The Principality of Zeon in Mobile Suit Gundam. Despite the misnomer (the head of state's official title was Sovereign), it practised all the human rights abuses and genocidal tendencies seen in any given "Evil Empire". The twist, however, is that they played the victim card throughout and made it seem like they were forced to declare war on the Earth Federation to gain their independence; in fact, the One Year War is known as the Zeon War for Independence on their side. And if that wasn't bad enough (since when does one invade other countries to declare secession?), the truth of the matter is Side 3 was already an independent republic under the previous founder Zeon Zum Daikun, the same man that the Zabis (specifically the patriarch Degwin) assassinated to subvert the Republic of Zeon into a totalitarian absolute monarchy. Overall, the Principality of Zeon is definitely the "evil empire" of the Gundam universe, in spite of having legions of diehard fans that would say otherwise.
The Zanscare Empire of Victory Gundam which was bigger, stronger and more brutally oppressive then even Zeon was - their main goal was a total cleansing on the Earth Sphere (presumably so that their Jovian backers can move in) by telepathically devolving humanity to a general infantile state. Notably, execution (such as by guillotine) was a standard punishment for failure in their ranks.
The Crossbone Vanguard and the Jupiter Empire of Gundam F91 and Crossbone Gundam. The Crossbone Vanguard was essentially a pirate themed Zeon-wannabe militia which sought to establish an aristocratically ruled dominion in Side 4, Cosmo Babylonia. Unlike Zeon however, it actually succeeded, but Cosmo Babylonia ended up collapsing on its own. Beside them, the Jupiter Empire, being more or less a precursor to Zanscare, sought to eradicate all life in the Earthsphere and then rule over the resulting wasteland. It also practiced many of Zanscare's tendencies, such as executions being commonly used for punishment, even for the most minor of crimes.
OZ, and latter White Fang of Gundam Wing play this role. OZ overthrew the corrupt United Earth Sphere Alliance so the world could be ruled by the aristocrats of the Romefeller Foundation, and then stamped out even minor rebellions through overwhelming force (namely in the usage of Mobile Dolls). To counter them, White Fang was formed in order to free the space colonies from their influence... by plotting to devastate the Earth with the space fortress Libra.
ZAFT / PLANT of Mobile Suit Gundam SEED. While they do have reasonable cause to go to war with the Earth Alliance, they tend to get pretty dirty - creating a energy crisis, troops murdering POWs, and generally being ruthless in battle. And it get's worse when the radical faction takes over the leadership and escalates the war.
The Human Reform League in Mobile Suit Gundam 00, rival to the Union and AEU, and seems to be based on both Communist China and the Soviet Union. Out of the three superpowers, the HRL seems to be the worse behaved: Human experimentation on refugee children, supporting a militant group to gain control of Ceylon. They're also the most antagonistic towards Celestial Being. On the upside, the League managed to avoid the worse excesses of Zeon.
And with Mobile Suit Gundam AGE, the Vagan have since joined the ranks of Gundam's many empires. Unlike most Gundam villain groups, the Vagans actually have a sympathetic reason backing them up: their nation began from Martian colonists who had been abandoned by the Earth Federation of the Advanced Generation universe and forced to endure living in a radioactive environment, resulting in much suffering and death. As such, the main drive for the Vagans is to escape their hellhole and return to Earth; unfortunately, they also want to exterminate Earth's native population along the way, as they see them akin to a disease that plagues their "Eden". And so just like the Zeon before them, the Vagans have no qualms committing genocide left and right, deliberately targeting civilian areas, even colonies and settlements with no Federation military presence, just For the Evulz.
CONSENT from G Saviour. They're pretty much what happens if you combine the Earth Federation with the Principality of Zeon.
An aversion of this trope is the Lyzelle and Elmekia Empires of Slayers. The latter isn't even visited in-series (although it's stated that it's Gourry's homeland), while the former holds two cities, one being a peaceful, bustling epicenter of trade, and the other is an otherwise peaceful village that hosts a magical tree with healing properties and gets blown up twice. If there is an evil country among in the Inner World, it's its only duchy, Kalmaart, home of the one city where a dreaded assassin lives, and (only in the novels) a village that worships the world's Dark Lord.
Amestris in Fullmetal Alchemist. It's stated that over the past 400 years, Amestris has conquered numerous nations, and is currently at war with Aerugo, Creta and Drachma. The Amestrian government is currently planning to use the entire nation in a transmutation circle to raise an immortal army using the slain souls of the numerous soldiers who have died over the years in a plan to conquer its remaining neighbors. Or rather, this is what Father has led the Amestrian government to believe. In reality, it's the crux of his own bid for godhood.
The Humankind Empire Abh from Crest of the Stars. Their mode of operation is to forcibly take over planets that do not have faster-than-light travel, though oddly they do not interfere with those planets which have purchased the technology from others. In the novels this is explained by the fact that their empire began on an interstellar merchant ship, and that they still respect "vested rights". They do not seem to be overtly oppressive towards the planets they control, but they do not allow any starship not owned by the Empire to be armed or use faster-than-light travel technology. Then there's the fact that only the "space elves" in charge are able to command warships, though this is due mainly to their physiology, not overt racism.
The Hellas Empire from Mahou Sensei Negima! is a partial subversion, since the war was actually orchestrated by Omnicidal Maniac terrorists. However, they made it look like the empire started it, and the heroes fought legions of imperial soldiers and warships until they learned of the conspiracy.
Played Straight, then subverted in Zoids: Chaotic Century with the Guylos Empire. They get the fancier equipment and the more villainous characters who seek to sow strife, but while initially an antagonizing force, it is discovered that it is mostly through the manipulations of the high-ranking nobleman Gunter Prozen that caused war to break out and, once the rightful ruler is returned to power, the Guylos Empire becomes a vital ally to the heroes and a more benevolent force. It keeps the name "Guylos Empire", however.
The Digimon Emperor in Digimon Adventure 02 manages to conquer a good chunk of the Digital World, enslaving it's inhabitants via mind control rings. Once Ken undergoes a Heel-Face Turn, his empire effectively ceases to exist.
The Shi'ar Empire has mostly been an aversion, Depending on the Writer and on the perspective of the characters. To the X-Men, the Shi'ar Empire has mostly been an ally (except for when they're dealing with a corrupt official or a mad emperor), but to the intergalactic rebels/pirates the Starjammers they were definitely the Empire.
Sonic the Comic has The Drakon Empire ruled by Master Scholar head of the House of Knowledge, Sister Haggra head of the House of Magic and Emperor Ko-Dorr head of the House of War, also the creators of Chaos energy.
Vakudos from Hottie 3 The Best Fan Fic In The World certainly runs the Midnight Cage and his chain of over 150 conquered alternate Earths like one. His title is "Emperor" for crying out loud.
The Empire of Darkness in Hottie 4: Even Better Sequel
Queen Of Shadows: In the new reality Jade finds herself in, she's Queen of a Shadowkhan Empire that's slowly conquering Japan one island at a time — they've already taken over Shikoku, and are in the middle of conquering Kyushu when the story starts.
There's also Shendu's Dragon Empire, which has conquered about half of China and a portion of Southeast Asia. Though apparently, the Ben Shui Chosen One is keeping him from advancing westward.
Fallout: Equestria and its Expanded Universe has the Pegasi Enclave, which rules Cloudsdale with an iron hoof and which emerges from seclusion to conquer the Equestrian Wasteland as well.
Sonic X: Dark Chaos: The Demon Empire is an autocratic and top-heavy Hegemonic Empire that rules half of the entire universe. The Angel Federation is the only thing keeping the Empire from ruling all of it. Slavery is legal (although heavily regulated) and Demon law is both harsh and unbending. Despite all of it, it's considered the closest thing to a unified culture the universe has; nearly every world uses Demon kredits, even if they live far on the frontiers.
Ages Of Shadow: Under Boaz's leadership, the Shadow Walkers emerge from hiding to begin carving out a kingdom in the Middle East known as the Himinate (after Boaz's title, Himinion). It expands for several decades, until King Cheherazad, backed by Persia, manages to take down the Shadow Walkers' leadership, scattering the remainders.
One Nation Earth in the Apocalypse film series by Cloud Ten Pictures. The design of their logo is based on the Eye Of Providence pyramid design of the Great Seal of the United States.
The United Citizen's Federation of Starship Troopers is this, being a Federation In Name Only. Their supercilious self-image just means they have to conquer the Klendathu Arachnids as revenge, who would actually qualify as the Empire themselves if they weren't the ones being invaded at the time.
The Persians in 300 are elevated into a dark realm built upon mass slavery and imperial deification, and are out to conquer Greece because Xerxes apparently just felt like it. Mostly they're an endless horde of Mooks for the Badass Army of the Spartans to fight, with the occasional monster ninja and demonic executioner here and there.
The Confederacy in Abraham Lincoln: Vampire Hunter. The vampires effectively control the south, using the slave system as an endless supply of food. With Adam's death and the end of the slave system, the empire collapses and most of the vampires flee the country.
The Radch in Ancillary Justice. Its economy is structured around a state of constant expansion, its populace is monitored constantly (though on a planet it's unlikely someone will be paying attention to an ordinary person at all times), dissidents get "reeducated", and it has a habit of turning people on newly absorbed worlds into meat puppets for their AIs at the slightest excuse.
Saruman attempted to set up one at Isengard, but failed.
Númenor, especially at the end of the Second Age. Númenor began as The Kingdom, but transmuted into the Empire - and was mighty enough to even conquer Mordor and humiliate Sauron.
The dominant human good guy societies, Gondor and Arnor (offspring of Númenor's uncorrupted minority) are also empires. Gondor at its height controls most of the southern reaches of the area mapped by Tolkien. Later, Aragorn, reunited Arnor and Gondor and establishes the Reunited Kingdom - a very large, civilized, and decent empire.
In David Wingrove's Chung Kuo series, the world is run by the Seven and called Chung Kuo.
The Galactic Empire in Isaac Asimov's Empire and Foundation novels is a state which has already achieved this goal in the distant past - by the end of the Foundation series, the Foundation is on its way to achieving the same. Partially subverted in that Asimov doesn't treat these either of these political entities as overtly villainous.
Asimov's Empire is explicitly modeled on The Roman Empire, and the Foundation's therefore bears some resemblance to Byzantium, the Greek-centered "Eastern Roman Empire" that lasted a thousand years longer - but not exactly, as it was set up specifically to reestablish the Empire within a single millennium.
Although the Terran Empire in Marion Zimmer Bradley's Darkover novels didn't start this way, by the time of its collapse it was a full-blown example of this trope.
The Empire in Terry Brooks's Shannara series is actually called The Federation. After refusing to get involved in the first three Shannara books despite the immense threat posed by the antagonists of each installment, they decide to invade the rest of the world in the Scions of Shannara multi-part series and (unintentionally) assist the real Big Bad in destroying the Four Lands. What a bunch of dicks.
The Barrayaran Empire and the Cetagandan Empire of Lois McMaster Bujold's Vorkosigan Saga both qualify at any point they're feeling expansionist. Barrayar's at the peak of one, and starting to slide down, during Shards of Honor and Barrayar, but by the time of The Warrior's Apprentice, it's mostly caught up colonizing a new world (one it didn't have to conquer; it was uninhabited by sentient species) and helps fight off the Cetagandans, who are still messing about. By A Civil Campaign Cetaganda's calmed down too.
Persian, Ottoman, Hapsburg, and Chinese influences all show up in the Imperium of the Dune novels. While it doesn't receive a villainous portrayal (indeed, the heroes run it in some books), it doesn't exactly receive a heroic one either. It appears mainly as a status-quo form of society that keeps working because it's what the characters can build in their circumstances (particularly the civilization's dependence on the Bene Gesserit and Spacing Guild, who both rely on a natural resource found only on one single planet).
Tanya Huff's short story "A Woman's Work" showed a well-maintained, organized and competently led empire, all thanks to a Queen who very clearly has the Evil Overlord List memorized (the story plays it for a comedy, with some direct references to the list).
Robert Jordan's The Wheel of Time introduces the Seanchan Empire as a civilization with its stability dependent on enslaving natural sorcerers channelers, and a universal delusion that the slave-masters are not themselves capable of channeling. Aside from the slavery and aristocracy, their strong socialist policies make them fairly popular among the conquered.
The slave-master part is partially justified as if they hadn't ever had a slave, they would be incapable of channeling.
Shara is another empire nearly as big as Seanchan on the opposite side of the world; it's mostly alluded to throughout the series without really being described, but supplemental materials paint of picture of an extremely insular, oppressively regimented society that's arguably even harsher than Seanchan, though far less expansionistic. In the last book, one of the Forsaken shows up at the Last Battle with a Sharan army in tow.
The Eastern Empire in Mercedes Lackey's Heralds of Valdemar series. A mild subversion, as the Empire is only evil at the top levels of the bureaucracy - when the armies wash over your country and start building roads, adequate housing, and utilities, most people figure they know what they're doing and don't put up a fight.
The Empire in Christopher Paolini's Inheritance Cycle, which consists of a single country ruled by a king.
One of the few "bad" things in Marge Piercy's Mary Suetopia in the novel Woman on the Edge of Time is that they still have war, and their agrarian, utopian society is fighting a war of attrition with an evil technocratic, cyborg, robotic empire. But, they must not be doing too bad, since while their utopia takes up most of the Earth, the evil empire is resigned to Antarctica, space, and the moon.
The CoDominium (CD) in Falkenberg's Legions by Jerry Pournelle. Technically The Alliance, consisting of the United States and the USSR, the CD becomes an interstellar empire, with Earth's nations really disliking the Alliance. It's also collapsing, because the US and USSR still hate each other.
Eventually the CoDominium military forces leave the Earth when the USA and USSR blow each other to pieces (and other nations join in), to relocated on the planets Sparta and St. Ekaterina, where they found a full-bore aristocratic empire of their own. The last line of 'The Prince of Spartans' is "AVE. AVE LYSANDER, IMPERATOR".
Inverted, played with and otherwise Deconstructed in the Honor Harrington series. Manticore may finally revel in some good old-fashioned imperialism, but that doesn't stop them from being the nice guy of the series, while alleged Federations are either corrupt bureaucratic monstrosities that are falling apart at the seams (League), or alternate between that and bloody tyranny (Haven). Although Haven has gotten a lot better lately. You can make a pretty good case that are now as much the "good guys" as anyone else. Right now the only reason they're fighting Manticore is over a really big misunderstanding (details would be a major spoiler). Indeed, the only true black in the series now are the people behind genetic slavery — everyone else is various shades of gray.
In C. S. Lewis's Chronicles of Narnia, Calormen is a subtle instance. First introduced in The Voyage of the Dawn Treader as "a wise, wealthy, courteous, cruel and ancient people," they are also so heavily into the slave trade that the Duke Bern predicts war with Calormen as a result of Caspian ending slavery in the Lone Islands. Although this war is apparently averted (it is not mentioned in The Silver Chair), the prequel The Horse and His Boy details Prince Rabadash's attempt to conquer Archenland, with a long-term eye on Narnia, some thousand years before. The curse on Rabadash is explicitly described as making life easier for small countries nearby, as he cannot conquer them himself, and is afraid of the power generals would amass if they did so for him. Of course, as The Last Battle details, Calormen does eventually conquer Narnia, but their triumph is short-lived.
Both subverted and played straight in The Malazan Book of the Fallen. The Malazan Empire is aggressive and expansionistic, but they're not evil. In fact, in the third book one of the leaders of the forces fighting against them comes to the conclusion that many of the cities they'd conquered were better off under the Malazans than they had been under their previous rulers. The Letherii Empire, on the other hand...
And the Pannion Domin, a theocracy of cannibals, is worse than Lether, to the point that most of the Domin is dead wasteland with only the fringes still supporting life. Fortunately, its also the smallest and least powerful of the three major empires of the current timeline (Malazan, Letherii, and Pannion Domin).
The Malazans may not be "completely" evil, but they are far, far from a strict subversion of this Trope, with their conquering and assimilating every single tribe that they encounter into their own religion and way of life is pretty damn evil. However, as is said above, their enemies are Not So Different.
Played straight and subverted in Mistborn: The Original Trilogy. The Final Empire controls the whole world and is a truly terrible place to live but its leader, rather than being power-mad, is a Well-Intentioned Extremist trying to protect mankind from an Omnicidal Maniac. After killing him and throwing the world into chaos, the heroes wind up having to create an empire of their own to stand up to the real Big Bad.
Inverted in David Weber's Safehold series, where the Empire of Charis is created by the protagonists. This is done out of survival since the Church of God Awaiting was trying to annihilate Charis and has been preparing steadily for another go after the first attempt failed. The Harchong Empire from the same series plays it straight though, as it practices outright slavery.
The Dark Empire of Granbretan in Michael Moorcock's Hawkmoon books.
In The Chathrand Voyages, two of these form the center of the storyline; Arqual is fairly progressive and cosmopolitan, but also aggressively expansionistic, while its Arch-Enemy the Mzithrin is an insular, theocratic regime. Neither empire is exactly what you'd call "nice"- we see more of Arqual's oppression of its subject peoples up close (because many of the main characters are Arquali or from Arquali-occupied territory), but the Mzithrin have the dubious distinction of producing the Shaggat Ness.
The Kingdom of Witchland and its many vasall states in E. R. Eddison's The Worm Ouroboros.
An Exercise in Futility has the Kalharian Empire. They aren't so bad, really, provided they conquered you a long time ago.
David Eddings has played with the concept. The Belgariad and related works has the Tolnedran Empire, which is one in name only, in order to concrete its similarity to Rome, and the Mallorean Empire, which in the first series is nothing more than a source for countless cannon fodder and in the sequel is expanded and explored in greater depth, revealing a rich and diverse multiculture who almost universally serve the Big Bad only because they're forced to, and are freed from his grip by the end of the series without any major political upheavals. The Tamuli, sequel to the unrelated Elenium series, explores the Tamul Empire in a similar way.
The Empire of Nilfgaard in the Witcher series. An expansive power ruled by an iron-fisted emperor which routinely uses terror tactics against the enemies' populace, it is also the place of flourishing economy where Trains Run On Time, scheming wizards are rare, and there is less racial prejudice or religious fanaticism, though an occasional coup d'etat does happen.
The Earth Alliance in the History of the Galaxy series, despite the name, had all the makings of an empire. The President ran things by himself and didn't need anybody's permission to declare an all-out war against the colonies, justifying that they needed to offload extra population (a single city on Earth is mentioned to have a population of billions) and the colonies wouldn't go fo it (they never actually asked the colonies outright). Their way of subjugating a colony? No declaration of war, although the sudden nuking of two major cities probably counts as one, followed by an invasion. When the invasion is unexpectedly repelled, the fleet admiral has the planet so thoroughly nuked that it remains uninhabitable for the next 1000 years. What follows is the First Galactic War, with the Free Colonies desperately trying to hold their own against the much more advanced and militarized Earth Alliance. After 30 years, the colonists finally manage to defeat the Alliance, and only because one of the Alliance top brass has decided that the war should end and turned off many defenses in the Solar System.
The Harammins had their own empire, including two slave races. However, they deliberately contained themselves in a large star cluster, never expanding, remaining static for nearly 3 million years with rulers suffering from Immortality Immorality.
A non-evil variant in Harry Turtledove's Worldwar with the Race, to the point where they can't conceive of any other form of government but absolute monarchy. This belief is further reinforced by the two previous races they subjugated, also reptilians. So when they arrive to Earth (which they call Tosev-3), they are baffled by (among many other things) by the wide variety of forms of government present on this planet. The only word they can come up with these forms of government in their language is "not-empires". Voting is derisively called "snout-counting". The Race have a Manifest Destiny-like belief on a galactic scale. They see their Emperors as ruling the entire galaxy (if not universe) by divine right. Their "evilness" can be described with the glee they express when they slaughter human soldiers or drop nuclear bombs on cities. They only reason they hold off on using nukes is because they don't want to damage the planet, which they want to settle. They're also highly hypocritical.
Every major power is an Empire in Belisarius Series. The difference is not between Evil empire and good "something besides empire", it is between "that's just how humans arrange their affairs empire", and "Really evil empire".
Fred Saberhagen's The Empire of the East is about the war between the Free Folk and the titular empire. This one is one of the more thoroughly evil examples.
The Yeerk Empire in Animorphs, complete with conquered planets and slave races.
The Enterran Empire from The Diving Universe. The main character, Boss, keeps claiming that she doesn't have any particular desire to come into conflict with it; but the events of the novels cause her to keep butting heads with it again and again, and things seem to be inevitably marching towards all-out war.
Weavers of Saramyr: The Saramyr Empire, starting out as an expansionist monarchy, complete with genocide.
In Poul Anderson's "Time Lag", Chertkoi, which ruthlessly seizes planets to exploit. Though they argue for it on the grounds their population needs the resources, they also regard it as Might Makes Right.
Several examples from Star Trek that The Federation encounters from the outside, like the Dominion and the Romulan Star Empire, and the Mirror Universe version of the Federation itself, the Terran Empire (insignia pictured above).
The Klingon Empire in the original series qualified until they allied with the Federation. Prior to the Dominion War however, it temporarily reverted back to its original practices, namely with the conquest of Cardassia.
According to some non-canon sources, the Terran Empire arose from the American Empire, which was originally a much more militaristic and strength-based United States, before Teddy Roosevelt renames it and crowns himself Emperor. Interestingly, unlike a typical monarchy, the title of the Emperor is not hereditary.note As with the historical Roman Empire, which looked down on kings and never really made the "Emperorship" an office in itself until the Dominate. In fact, very few Emperors actually died of old age. Most were assassinated by those who then took up the mantle. Only one actually retired.
The Cardassian Union is an imperial military dictatorship, run by several military officials in governing body known as the Central Command, rather then one Emperor. It is more similar to the USSR or Oceania then a traditional empire, controlled by a political body. The Cardassian Occupation of Bajor has shown the Cardassian Union to be particularly brutal.
The Federation itself is compared to one in-universe, with several characters pointing out that despite their outward appearance of benevolence, there's something incredibly insidious about how they worm their way into situations and make it so that other races come to need them, while others point out how they're Not So Different than the Borg, since they assimilate all kinds of people and cultures while no-one even notices.
The Alliance of Firefly, though since the main characters are all anti-heroes, it's suggested that from another point of view the Alliance might be considered The Federation. However, the Academy, which is a subset of the Alliance, is firmly on the side of evil.
And then The Movie came and the Alliance crossed the Moral Event Horizon with what went down on Miranda. At least not until the Operative shows up on Serenity's dock with a Heel Realization to get off his chest, suggesting that even the Alliance has members too moral to tolerate that, and isn't unanimously 'evil'.
The Scarrans and the Peacekeepers, mortal enemies in Farscape. They're both bad enough that Crichton feels that destroying the entire universe is a better option than allowing either side to gain supremacy over the other.
The Dalek and Sontaran empires in Doctor Who, along with basically any Empire. Subverted in the Classic Series with the Draconian Empire, which is more The Kingdom than anything else. The portrayal of the Earth Empire varies.
The Federation in Blake's 7, despite the sarcastic name (its logo is the Star Trek Federation's, given a hard twist to the right).
In Andromeda, the Nietzschean plan during their rebellion against the Systems Commonwealth was to form a strong Nietzschean Empire with the Drago-Kazov pride forming the ruling dynasty. However, during the last major battle of the war, the Battle of Witchhead, a large Nietzschean fleet (constituting 15% of their total strength at the start of the conflict) suffers heavy losses when they destroy the last High Guard ships (with some help from Hunt). No longer having a strong position, the Drago-Kazov pride is unable to stay in charge, reducing the unified Nietzscheans into a bunch of squabbling prides who are more concerned with power than creating civilization. Cue 300 years of total chaos.
The Commonwealth itself originally started as the Vedran Empire before it became more egalitarian and switched to a republican form of government.
In Stargate SG-1, the Goa'uld Empire was an evil empire in name only (the "empire" part, they're a proudly evil bunch). Territorially it was the most powerful grouping in the Mily Way Galaxy, but in fact it is deeply fractioned between numerous regional System Lords who constantly fight each other for supremacy and after the death of The Emperor Ra in the original movie any semblence of unity is long gone. This initially worked in the humans' favor so they could focus on taking out each threat one by one, but frequently one of the Goa'uld would emerge victorious over the others to fill the Evil Power Vacuum and attack Earth directly.
The Coalition States in Rifts can be given a little slack for establishing order in the post-Cataclysm world... but not for enforcing illiteracy, destroying pre-Rift artifacts, and brutally hunting down and persecuting D-Bees, magic users, and psychics with chilling coldness.
And Emperor Prosek's decision to consciously adopt Adolf Hitler as his role model really doesn't help. Although Prosek is not nearly so racist or anti-magic as he pretends. He just finds it useful for keeping the public simultaneously scared and ready to fight at his command. Also, the simple act of him successfully declaring himself "emperor" the Coalition States is in line with this trope, as they were originally more democratic than they are under his rule. This is a major source of tension between Chi-Town and Free Quebec, and also causes concern on the part of the New German Republic.
Rifts is full of these, mostly ruled by monsters. Atlantis, the Gargoyle Empire, the Vampire Kingdoms, the Phoenix Empire, etc.
Warhammer 40,000 has two major examples. And those are, by comparison, the good guys.
The Imperium of Man is notable for it's sheer scale, A million worlds and with a population in the quadrillions, it fights thousands of wars at any given moment, its soldiers number in the trillions, and it has existed for ten thousand years. One man, even the greatest man, is nothing, even the death of millions matters little in the Imperium.
The Tau, who are also The Federation made up of many different races including human turncoats while the Imperium wipes out all alien life they find.
Various Chaos Warlords (both human and not-quite-humans-any-more) also sometimes manage to carve out they're own mini-empires, usually numbering a few dozen or hundred worlds. These are either eventually crushed by an Imperial Crusade or collapse into anarchy because, you know, they're chaos...
The Skaven fit the trope better, they have a massive underground empire that dwarfs the Empire, and all the other Old World nations combined. They have the potential of taking over the Old World, if they would put aside their Chronic Backstabbing Disorder for one minute and not try to eliminate each other while fighting their enemies.
The Third Imperium of Traveller is more like The Federation with hereditary nobles than an example of this.
The First Imperium when the Terrans contacted it in the Interstellar Wars(which is as far back as Traveller sourcebooks go) was a Vestigial Empire. The second was essentially the first following conquest by the Terrans but the size and senility of the first Imperium simply overwhelmed their bureaucratic resources causing the Imperium to collapse entirely almost as soon as the end of the Interstellar Wars.
Inverted in Anima: Beyond Fantasy with the Empire of Abel and the Azur Alliance -as well as before things got screwed up, when the Empire is described as a benevolent entity (at least for humans and not supernatural stuff)-.
In BattleTech, the Draconis Combine fits this to a tee. In fact, it's repeatedly stated in the sourcebooks that the Kurita family (who rules the Combine) believes that it is their destiny to rule all of humanity one day.
While the Star League acted like The Federation when dealing with the Inner Sphere its treatment of the Periphery states was less than charitable. They were conquered in a brutal invasion and then heavily taxed and denied a say in League policy.
All of the Successor States are empires. The Draconis Combine is less subtle about it than the others, but all five are cut from the same ruthless cloth and are ruled by dynastic emperors. Their primary opposition, the Clans, are The Horde, and the closest thing in the Inner Sphere to a democracy, the Free Rasalhague Republic, got conquered by Clan Ghost Bear. Though the Clan did allow Rasalhague to retain their parliament and Elective Monarchy, however Princes tend to be Ghost Bear Khans ever since they claimed Prince Ragnar as a bondsman.
Recently Clan Wolf has renamed itself into the Wolf Empire thanks to an alliance with a branch of House Steiner.
Final Fantasy II has the Empire of Palamecia, which conquers, destroys, and enslaves seemingly for the heck of it. Partway through the game the Emperor kills the devil and usurps his throne. And as if that weren't enough to make him a badass, the expanded GBA release reveals that his "good" side has also taken over the equivalent of Heaven.
Final Fantasy VI has an empire called...well, "The Empire" (it was renamed "Gestahlian Empire" in the GBA release) as the main antagonist for most of the game. As if it weren't clear enough, the intro shows the emperor giving the Nazi salute to his subjects. They're not the dangerous ones.
In Final Fantasy IX, Queen Brahne is in the process of creating one of these until her death.
Final Fantasy XI includes the Empire of Aht Urghan, which makes up the setting for it's named expansion. It rules over the Aradijah continent with an iron grip, and is in constant war with various Beastmen factions and the Far Eastern army. Bioweapons, chimeras, electric harnessing and Blue Mages are the result of Aht Urghan's technology. The best part is that the player character works as a mercenary/double agent for the big bad Empire for the entire plot.
Final Fantasy XII has the Archadian Empire, which embodies this trope, and the Rozarrian Empire which seems more benevolent, but is only peripherally involved in the story.
Final Fantasy XIV brings us the Garlean Empire, the belligerent invading force against Eorzea and the primary antagonists of the game's main scenario. Thematically, they borrow heavily from FFVI's Gestahlians (including explicit use of magitek), as well as the Imperials from Star Wars.
The Suikoden games have the Scarlet Moon Empire and the Harmonian Empire.
While not always directly antagonistic, the Holy Lodis Empire is easily the largest military force in the Ogre Battle and Tactics Ogre games and makes its presence known similarly to the Harmonians listed above.
Alfard from the Baten Kaitos video games is an interesting example in that there's no resistance against it. The reason: the empire works to instill extreme civic pride in all of its citizens, so that the idea of acting against it has all the attractiveness of stepping on one's own foot.
Lyude: It's unthinkable! To act against the Empire!?
In Panzer Dragoon, the Empire is portrayed this way, but slowly becomes more and more sympathetic as the series progresses.
Spiderweb Software's Exile / Avernum series starts the first game with a description of The Empire. "Not the Empire of Something or The Something Empire"... since there's really only one game in town. They find a massive underground cave network and decide it's the perfect place to chuck all their undesirables. Including you. So the rebellious elements in society are rounded up and sent down to live in near-darkness and plot revenge. And exterminating all non-human species on the surface.
One of three ways to win the game is to join the rebels and help take revenge by assassinating the leader of The Empire. Although this does spark a war for the second game, and, well, The Empire does have a few more legions of disposable soldiers than the rag-tag underground rebels do. (Good thing you find aliens to help you!) Seriously, this game is fun.
Subverted in the third game, when Avernum ends up helping the Empire out. This leads to the Reconciliation and the Empire and Avernum becoming allies. The fifth game even has you play as Empire soldiers.
The Elder Scrolls series contains an unusual subversion, in that the Empire is generally treated as fairly benevolent. By Skyrim, they've lost a lot of power and have become somewhat corrupt since their founding dynasty expired, but they are still more benevolent than most examples of this trope. The Aldmeri Dominion plays this trope straight.
Mage Craft The Altan Empire fits this nicely. They're even crushing a nearby republic.
If it's a Fire Emblem game it has one of these. Generally the Empire has somehow fallen into the power of some dark evil God. Fire Emblem: Path of Radiance's Daein and its King Ashnard are the best fitting, the others are all 'correct' but with a few details of note.
The sequel, Radiant Dawn, gives this role to Begnion, which, unlike Daein, actually is an empire. The Dawn Brigade of Daein, the Greil Mercenaries of Crimea, and the empress of Begnion end up fighting against the empire's corrupt senate.
Mystery of the Emblem offers an interesting case - the Holy Kingdom of Archanea was a steadfast ally during the previous war. However, when Hardin ascended to the throne, he rechristened it the Holy Archanean Empire, changing practically nothing else about it save for its motivations; it's the antagonistic nation of the game.
The Covenant is a theocratic empire of multiple alien species lead by religious zealots on a crusade to assimilate every race in the galaxy and reveal the divine truth of the Forerunners, whom they revere as gods. The Covenant teaches that by recovering extant Forerunner technology ("holy relics"), they will discover the locations of the Halos ("sacred rings") and obtain the means to embark upon the "Great Journey" - following in the path of the Forerunners and becoming gods themselves. In the year 2525 AD, the Covenant embarked on a holy war against Humanity, proclaiming the entire race to be an abomination to the gods. In reality, the true motivation behind the war was to erase all evidence indicating a special connection between Mankind and the Forerunners. The Covenant came dangerously close to achieving its goals, pushing the human race to the brink of extinction over the course of a protracted interstellar war and obtaining the means to activate the Halos, which in fact had no function but to kill all life in the galaxy. However, at the climax of the conflict the Covenant was torn asunder; shattering into warring factions upon the betrayal of the Elites by the Prophets and the concurrent revelation that there was no "Great Journey". The UNSC was able to forge a temporary alliance with the forsaken Elites and defeat the orthodox Covenant, securing the continued survival of Humanity.
The United Nations Space Command is the military, scientific, and colonial administration agency of human space in the twenty-sixth century. The UNSC is viewed by many of its own citizens as an oppressive, totalitarian empire, and indeed had its hands full fighting the Insurrectionists (though they are portrayed more as fanatical terrorists and mass-murderers rather than romantic freedom fighters). By the time of the first game, most of the Insurrection had been wiped out by the Covenant and the UNSC had tightened its control over the surviving worlds, with the central civilian government ceding to it most of its remaining powers. With the dissolution of the Covenant, the UNSC is currently the dominant interstellar power in the Orion-Cygnus Arm (despite an Insurrectionist resurgence), as demonstrated by the commissioning of the Infinity and the SPARTAN-IV program.
As shown in The Forerunner Saga, ancient humans, in conjunction with the San'Shyuum (Prophets), also had their own interstellar empire during the age of the Forerunners, and ended up warring with them during their expansionist efforts. Although nearly advanced as the Forerunners themselves, the humans and their allies were out-numbered, out-gunned, and out-maneuvered by their adversaries. The Forerunners defeated the humans and obliterated their civilization, deprived them of their technology and worlds, erased their culture and history, and exterminated most of their population - the remainder were subjected to biological regression and fragmented into numerous sub-species before being forevermore sequestered on Earth as a nature reserve and laboratory animal source. The San'Shyuum got off comparatively lightly due to surrendering first, and were "merely" sequestered to two planets. Then the Forerunners realized the humans were not trying to conquer them, but instead were attempting to flee from the encroaching Flood.
The latter being an allusion to the real-life Second Triumvirate. The Triumvirate has no territories it controls, just a network of agents and blackened ruins where they have attacked. It is explicitly stated they don't care who rules as long as the Jedi are wiped out, since they would be unchallenged to do anything they want.
Both of those pale in comparison with the True Sith Empire, hiding in the Outer Rim, gathering strength to attack the weakened Republic. In fact, according to the novel Star Wars: The Old Republic: Revan, Revan is the only reason why the Sith Emperor doesn't immediately strike at the Republic, which, thanks to Revan himself, is quite vulnerable to attack. Instead, he manages to keep the Emperor doubting for 300 years before the attack finally happens.
But given a first-rate Deconstruction in Star Wars: The Old Republic. The Empire has the infrastructure and political stability of a third-rate Banana Republic. Their "capital" is about the size of a Republic military base (compare to Coruscant, an entire city-planet) and doesn't even have paved roads to the nearest spaceport, meaning all trade and military equipment have to slog through a predator-infested jungle just to get to the city gates. The Sith are too busy with their internal power games to pay any attention to governing, and frequently spin off into their own little feifdoms, wasting time and resources the Empire doesn't have. Everyone from the Sith to two-credit bureaucrats and petty officers are trying to undermine each other to gain greater individual power instead of the war effort. Add truckloads of Fantastic Racism (anyone not human or Sith species is only good for the slave auction, unless they're Force Sensitive, which sends them to the Sith), and an untenable reliance on slave labor with the slaves in a constant state of revolt (and they're building monuments to the Sith's egoes, not infrastructure). And the Emperor himself? He just wants everything aside from himself to die so he can become a god. They had three centuries to stockpile, waged an effective shock and awe attack since the Republic didn't even know they existed thanks to Revan's stupidity, and have pretty much lost it all in two decades. It's no wonder that, come the Makeb storyline, Darth Marr admits the Empire is screwed.
Despite the name, the United Earth Federation of Supreme Commander is very much an empire, and a military dictatorship at that. Slavery of sorts is practiced, and there's at least some degree of restrictions on political freedom-the Infinite War began when outlying colonies began rejecting Earth rule and joining the Aeon. Earth's response was military in nature, one thing lead to another, and the Infinite War kicked off.
The Aeon would also count, since they happen to led by a warmongering commander who plans to usurp the princess, and their goal is to brainwash everyone into accepting The Way, and cleanse all nonbelievers.
A few corrupt officials and officers aside, this is generally averted in Tales of Vesperia, where the Empire is not the primary antagonist.
Secret of Mana has an empire simply called the Empire that wants to harness the power of the Mana Fortress. A group in one of its towns was formed to work against it.
The Dilzweld Empire. Their army seems unstoppable— at the start of the game, they are about to attack The World Alliance (Huh, kinda sounds like The Federation, don't it?), the grouping of the other five mighty nations, and they are certain they can win. Although, they might have some trouble with that, considering whole platoons of their gun-packing Army (they have mechs and airships, too) are defeated with little difficulty by a group of adventurers armed with a sword, an axe, a bow, and some kind of barbed fishing fly on a string.
The Romalian Empire were actually somewhat competent and are the cause of all the problems in the series. The Academy from Arc the Lad 3 fit this trope too, albeit they are a collection of scientists and scholars.. with platoons of soldiers with heavy machine guns and plans to rule the world. Romalia is a subversion: it started as a commercial empire, because a Romalian merchant was the first to discover spirit stones and to use them as an energy source: When Arc the Lad start, Romalia has been the world first super-power for already 1000 years, thanks to its control of the energy supplies, and things started to go really bad only one generation ago.
FreeSpace has the Ancients' Empire in its Back Story, which fell 8,000 years before the game begins, annihilated by the Shivans. The Back Story is told from the Ancients' point of view, chronicling their rise and fall, painting them somewhat sympathetically as victims of the "Cosmic Destroyers"... it's easy to forget that they were a galaxy-spanning war machine conquering planets and enslaving species left and right: "And we saw other advanced life, and we subdued it, or we crushed it..." The Shivans could almost be seen as the heroes in this case.
The Vasudans are also governed by an Empire, though it seems to be a fairly benevolent one. It's also specifically mentioned to be a Parliamentary Empire. Indeed, the Vasudans' survival and economic prosperity following the devastation of their homeworld is largely attributed to their current Emperor, who is quite progressive and favors further integration with the Terrans in the sequel.
Super Mario Bros. has the Bowser Empire/Kingdom, or whatever it's called. Heavily militarised, run by a despotic monarch and frequently invading the Kingdom... but oddly, in later games it's shown that not only are Bowser's soldiers mostly just doing their jobs, they actually like, respect and admire Bowser, and are quite loyal and proud (if not understandably scared of pissing off the giant super-strong turtle-dragon king).
Inverted with the Lanvaldear Kingdom and the Malkuth Empire in Tales of the Abyss. Through Luke's POV, the Malkuth Empire should be the bad guys and his own kingdom Lanvaldear is good. Turns out, neither is really good nor bad. If anything, Malkuth is shown to be much more benevolent.
Played quite straight in the MMORPG Pardus, where the Empire has won every war it was involved in up to this point(no, I'm not talking about the backstory either). This is even more surprising when you consider that the Empire(along with the other factions, The Federation and the Union) is completely player run!
However, that is in orion, one of its three universes (servers). In Artemis, the empire loses quite often, with the federation mostly winning due to having the most members and the best organisation. Then again, wars are not really common, being mostly once a year.
Overlord II has the Glorious Empire, a copy-paste of the Roman Empire, seeking to eradicate all magic in the world. By "eradicate", The Emperor means gather it all in one spot then use it to become a god and reshape the world. Standard stuff.
Averted in the Disciples series. The Empire, the human force and the local Jack of All Stats race, are more often than not generally the good guys, though there were some corrupted nobles that gained a lot of power during the time skip between the Original game and the Sequel, because the Emperor did mostly nothing for the entire decade. Once they got a new King, they cleaned up their act considerably. And then the Elves invaded them for poorly explained reasons.
The Auroran Empire of Escape Velocity Nova is mostly The Alliance merged with Proud Warrior Race, but they do have their Imperial moments and aspects. The real example in Nova is the Bureau, working through The Federation - they are an autocratic organisation (headed by a Chairperson) seeking to suborn all humanity under their rule, and already have control of one of the three major states of Known Space.
The Terran Dominion, ruled by Emperor and former terrorist, Arcturus Mengsk.
The UED in the Brood Wars expansion.
The Terran Confederacy was this in all but name, as well as lacking a single ruler. Their most famous acts are nuking a planet for some of the citizens speaking out against them and sending assassins to decapitate a family. No wonder many humans actually welcomed Mengsk's new government.
The, well, Empire in Gratuitous Space Battles, which is one of the most powerful galactic civilizations, a technologically-advanced powerhouse that has existed for thirty thousand years and counting and is bent on conquering what parts of the galaxy they haven't taken yet. Their "Emperor" is a bit "under the weather" - no one's seen him in about a thousand years or so, but the Empire itself keeps marching along.
The Galaxy-spanning Taiidan Empire in Homeworld can perhaps best be described as what would have happened if the Galactic Empire from Star Wars had been allowed to remain intact for 4000 years. Not very pretty.
The Back Story reveals that the Hiigarans were pretty imperialistic in the old days, boldened by the fact that they had one of the three original hyperspace cores, giving them immense power. They use it to transport their fleet past the Taiidani defense lines right above the Taiidani homeworld, raining destruction on it. After the Bentusi interfered and took away their core, and wiped out a sizable chunk of their fleet, the vengeful Taiidani struck back and drove the Hiigarans from their homeworld, which they took for themselves as repayment.
Caesar's Legion in Fallout: New Vegas, modeled off of Imperial Rome, have the Evil part down, adopting traditions of cultural extinction and genocide, slavery and crucifixion. Their objective in the game is to establish the Empire part. Caesar however still thinks they're The Horde, and wants Vegas to be their Rome thus making them a true Empire.
Eien no Aselia has the Sargios empire, the strongest nation on the continent. The people are actually perfectly content since they don't have to do any of the fighting and are benefited by the imperialistic nature of the empire.
The Junaris Empire of Vanguard Bandits. The largest nation on the continent, they came into power by rebelling against the Pharastia Kingdom and continue to war with them to the current day. They're the most consistent enemy on every path in the game.
The Empire of the Rising Sun in Red Alert 3. All hail Emperor Sulu!. Interestingly, if you win the game as the Empire, Emperor Yoshiro grants you the rank of shogun. Historically, the shogunate usually held all the power in Japan, while the Emperor was merely a figurehead.
The Terran Empire in Ground Control II, originally the Draconis Empire, who renamed themselves after conquering Earth. They then spend centuries solidifying their hold on the Inner Sphere colonies and building up their military. They then strike out at the Northern Star Alliance, crushing all resistance mercilessly. The leader of the Terran fleet is Imperator Vlaana Azleea, AKA the Butcher of Ariel, charged by The Emperor to bring all the colonies under Imperial control, even if it means bombing them into nothingness. The Empire is partly modeled after the Roman Empire, including the ranks (imperator, centurion) and the fact that their foot soldiers are called legionnaires.
Universe at War: The Hierarchy play the stereotypical role of an Empire. Their a galaxy spanning race that has conquered countless worlds, and strip mine them for their resources (and by resources it includes all of its living inhabitants).
In the X-Universe, the Split Dynasty and Paranid Empire are opposed to the Argon Federation and Kingdom of Boron. The Split are the straighter version in that they're the only member of the Commonwealth that still practices slavery. Still, morality in the series is kinda grey; overall the only thing that makes either side good or evil is that the last time the two sides went to war, the Split and Paranid were the aggressors.
Subverted in Naev. At the current stage of development the Empire is actually the nicest, most stable government in the setting.
As of Star Trek Online, the Klingon Empire has again reverted back to its TOS practices, once more dropping its alliance with the Federation and waging a war of conquest in the Alpha Quadrant. So far it has conquered Gorn and several other worlds, established military alliances with the Nausicaans and Orions and has made several incursions into Federation space. Overall it's now the second leading superpower in the ST universe after the Federation (discounting the Borg, Undine and Iconians).
Cerberus from the Mass Effect universe is a privately-funded human supremacist-elitist organization devoted to bringing about Mankind's ascension over the entire galaxy by any means necessary. By the time the events of the games take place, Cerberus has accrued a terrifying reputation as a result of its clandestine affairs; including nightmarish experiments on alien and human abductees, infiltration and subversion/destruction of rival entities, and the assassination of recalcitrant public figures and their subsequent replacement with more sympathetic voices. Naturally, Cerberus has been classified as a terrorist organization by the Citadel council in light of the group's anti-alien ideology and criminal activities. Cerberus exists as a rival and a foil to the legitimate human interstellar government, the Systems Alliance. The Alliance also regards Cerberus as a terrorist organization, although one plot thread states that Cerberus began as an Alliance black-ops section that went rogue. In contrast to the Alliance's parliamentarian system of elected officials bound by constitutional checks-and-balances, Cerberus is an omnipresent, totalitarian entity stratified into a network of physically-isolated, functionally-independent cells personally overseen by Cerberus' executive - the Illusive Man - who operates without any hesitation or restraint, save for the limits of his own knowledge base.
In Mass Effect 3, the Illusive Man comes to the conclusion that he possesses the means to harness the Reapers and their technology to bring-about his vision of a human-dominated galaxy. Cerberus steps out of the shadows and invokes the Empire trope, fielding an army of white-armoured, brain-washed slave soldiers supplied by planetary systems conquered and oppressed by Cerberus during the course of the Reaper invasion.
Mass Effect also provides more examples of imperial enterprise:
Rising from their homeworld after reverse-engineering the Reapers' technology - the Mass Effect - the Protheans forged a mighty galactic empire through the aggressive assimilation of alien species; those who submitted were allowed to prosper under their Prothean masters and even have a circumspect voice in interstellar policy. Those who resisted - such as the Zha'til - were exterminated completely as an example to others. The Prothean Empire was smashed to pieces when the Reapers poured through the Citadel Mass Relay, disabling the Mass Relay network and destroying the Prothean leadership in a single attack. The subsequent Harvest lasted for centuries as the Reapers moved at their leisure from one isolated system to the next, the Protheans powerless to mount an effective response with their logistics and communications severed. When the Reapers completed their harvest and retreated back to dark space, the dozen or so Protheans remaining alive made a one-way trip to the Citadel through the Conduit and sabotaged the space station's systems to force the Reapers to return to the galaxy by means of conventional FTL, giving the species of the next cycle more time to prepare for their arrival. A scattering of beacons across the galaxy contained messages meant to warn future space-faring civilizations of the existence of the Reapers, and the malevolent intentions they carried in-store.
The once-mighty Batarian Hegemony is a vestigial empire in an advanced stage of decay by the time humans appear on the galactic stage. Cruel and despotic, but otherwise corrupt and ineffectual, the Batarians are Humanity's primary extraterrestrial rivals in the latter's efforts to colonize space. Rather than jeopardize its dwindling military resources in an all-out war with the Alliance and risk the unilateral intervention of the Council races, the Hegemony has resorted to financially backing independent pirate and slaver attacks on outlying human settlements. In Mass Effect 3, the Batarian Hegemony is effectively obliterated as the very first victim of the Reaper invasion, thanks in large part to the Reapers having compromised the Hegemony's chain-of-command with their indoctrination measures.
In contrast to the previous entries, the Turian Hierarchy is an example of the good empire. A Council race and a civilization of professional soldiers; the Turians live, breathe, and die on the mantra that the needs of the many outweigh the few and that no personal sacrifice is neither too small nor too great for a cause greater than one's self. The Turians are the Mass Effect galaxy's peacekeeping force, holding the barbarians at bay and preventing the light of civilized life from ever being extinguished. The Turians also maintain a string of client-states on the periphery of their territory; races that have accepted Turian dominion in exchange for guaranteed protection and limited sovereignty. The Volus in particular provide necessary economic infrastructure that the Turians have always lacked. When humans made first contact with the Alliances' armed forces at Shanxi, the Turians chose to wage war and assimilate the humans into their empire as a client race. Their efforts backfired when their mobilization drive provoked an intervention from the Citadel council and the subsequent negotiations brought the conflict to a close and Humanity was inducted into the galactic community.
Wild Star has The Dominion, who span most of the known galaxy and intend to bring the "unenlightened" vermin of the universe to their glorious ranks... or crush them, if they choose to resist.
Led by the Visionary, the Authority seeks to establish a military dictatorship over the entire Earth in the early twenty-second century following a major impact event and is the Big Bad in Rage. According to the Word of God, the Visionary is an American Army general named Cross who hijacked a "Super Ark" bearing elite soldiers, technical specialists, and bleeding-edge, prototype technology. The Ark program was meant to ensure the continuity of the human race, but Cross has warped its goals in order to stamp his New World Order across the entire planet. To accomplish this, Cross imprisoned the remaining Arks beneath the Earth's crust - where their inhabitants would eventually perish inside their stasis pods when on-board life support systems failed. Then he built the largest, most disciplined military the Wasteland had ever seen. A rather easy task, as the Authority is the only entity that possesses the sufficient resources necessary to field and equip a professional army of armored soldiers with advanced weaponry.
At first, the Wastelanders welcomed Cross and his Authority as saviors; hoping-beyond-hope that they would crush the deranged cultists and bandit tribes and bring-about a new golden age for Humanity. In Cross' mind, he did just that; however his tyrannical methodology alienates the local populace from his cause and sparks a La Résistance movement - called the Resistance - led by disaffected Ark survivors who managed to make it to the surface and escape the Authority's routine sweep-and-purge operations.
The very first Heroes Chronicles features (the Empire of) Bracaduun, ruled by tyrannical Wizard-Lords, which at that point is the most powerful state in Antagarich, dominating the entire southern region and oppressing the barbarians of Krewlod. It gets toppled by the barbarian protagonist (who at that point has descended into being a tyrant that seeks his own barbarian empire). There are other empires scattered about the timeline, but they tend to have rather short imperial periods or very little information on what kind of empire they actually were.
Girl Genius: The Wulfenbach Empire, founded by Baron Klaus Wulfenbach during the chaos following the Other's reign of terror and based around the principle of "don't make me come over there!" Given that most rulers in the setting are Mad Scientists this generally means that Klaus is putting down a rebellion quite often, and few of them are good, possibly even the one our intrepid heroine finds herself forced into simply by being a lost heir to one of the more influential (and crazier) Spark families.
The worst part of this is that the Wulfenbach Empire is the most effective government in the known history of Europa. The last Empire on its scale was the semi-mythical Storm King's domain centuries ago. In fact, after a two year timeskip in which the Wulfenbachs have lost most of their territory from rebellions, the Empire is thought of as a lost Golden Age and many fiefdoms suddenly finding themselves under attack beg to rejoin. All that after just two years.
Homestuck: It's never seen in action being that the main characters are all teenagers, but the Alternian Empire from which the trolls hail was reportedly a formidable and dominating conquering force constantly at war across the universe, seeking to conquer and add more territories to its own. After hundreds - possibly thousands - of years of this, the Vast Glub and the meteor apocalypse on Alternia put an end to that.
The Souballo Empire from Our Little Adventure is probably going to be the main antagonistic force of the comic.The adventuring group is about to get on a boat to the continent where its holds a huge foothold.
Tech Infantry has several factions, both human and alien, that occasionally or always fit this trope. The Earth Federation was determined to be the only government of the human species, and aggressively expanded at the expense of various alien empires, who were trying to do the same to them. Then they get replaced by the Middle Kingdom after one faction finally wins the seemingly endless human civil war, and they are even worse in this department. Various alien empires, from the Arachnids to the Jurvain also fit the trope.
The Enforcers of the Plot Continuum, the PPC's Evil Counterpart, are dedicated to multiversal conquest and apparently already rule large sections of their multiverse. On finding out about the prime multiverse, it's been hinted they've now set their sights on it.
The Sith Council and The Galactic Empire are modern factions in The Gungan Council that have the most planets out of any other faction.
Every server of Imperium Nova has the player's houses operating in one, at least in name. Gemini has at least seven, plus an Alliance.
Pixiv Fantasia I: Aizerun. The empire holds an ideology that basically amount to fantastic fascism. A country of Beastmen used to exist in its southern territory. Used to.
Originally, Firelord Sozin (Ozai's grandfather who started the war) wanted to spread the Fire Nation's culture and prestige all over the world by establishing illegal colonies on the Earth Kingdom's land. After the genocide of the Air Nomads due to Sozin's fear of the reborn Avatar his descendants suffered extreme Motive Decay and more than a little Sanity Slippage.