Choirs of daemons howl in salute as they sense his flagship exiting from warpspace at the edge of the system. Chains of cities across the surface of the planet are set ablaze in welcome as his battle barge slides into orbit. Thousands of slaves are sacrificed in celebration as he sets foot upon the surface, the very earth beneath him soaked in the blood of his victims.
Generally below the Dimension Lord but above The Emperor and the Evil Overlord lies this guy.
Maybe he wants to conquer the universe. Maybe he wants to kill everyone in it. Maybe he just eats planets. Whatever he does, he'll be crazy-mad powerful, so it's going to take a whole boatload of heroes to take him down.
Sometimes he'll have an army to back him up, but more often than not, he'll work alone, relying on his incredibly vast cosmic powers to crush the good guys. Many have both.
These characters are usually male, although fortunately changing gender roles are offering more and more females the opportunity to enslave entire galaxies. Distinguishable from the Eldritch Abomination in that these guys have somewhat human motives for what they do. Often, this is what the Sealed Evil in a Can turns into once it's unsealed. Take this Up to Eleven, and you get a Multiversal Conqueror.
King Vega from UFO Robo Grendizer, one of the Mazinger Z sequels. He was emperor of a warrior race and ruled over a planet was dying due to overexploitation from one of its resources -a radioactive and highly unstable ore named Vegatron-, so he started to conquer other planets to find another world to settle on. He used to say his army was the most powerful one in the universe.
Freeza from the Dragon Ball series. Immensely powerful to the point of being considered invincible, he nonetheless usually has his army to do the actual fighting. Known for exterminating large portions of planets' populations and enslaving the rest, or simply killing off the entire population and selling the planet to the highest bidder, his name was feared by all spacefaring civilizations throughout the galaxy. The fact that he could blow up your planet on a whim just served as further incentive to surrender.
There's also Lord Slug who also fits this trope well.
Zoltar from Battle of the Planets (and only Battle Of the Planets. The original Gatchaman was set entirely on Earth, villains and all!)
Reinhard von Lohengramm, the protagonist of Legend of Galactic Heroes is a rare heroic example of this trope (although he might count as an Anti-Hero. Might.) He wants to unite all of space under his rule. The thing about it is that his Empire is actually pretty awesome because he's a superb ruler. They're opposed by the democractic Free Planets Alliance. A huge topic in the series is basically the conflict between a good autocracy and a rotten democracy.
Subverted twice: first, Yang Wen-Li, his rival from the Free Planet Alliance is a much better conqueror than him (a pissed Yang can fight Reinhard to a standstill with a tenth of his troops), the only thing stoping him from conquering the galaxy being his lack of ambition and distaste for playing political games; and second, the real Galactic Conqueror is not Reinhard but his wife, Hilda, the only character in the verse who actually manage to outgambit Yang
The aptly-named Sailor Galaxia from Sailor Moon. No points for guessing what is that last planet she needs to conquer to complete her quest.
And when we mean "Obsession with Death" we mean the Stalker with a Crush kind of obsession. Said girlfriend is Death.
This is a general characterization for most cosmic-centric supervillains within the Marvel Universe. Some prominent terran-centric supervillains like Doctor Doom and Kang The Conqueror occasionally branch off into this trope, when their ambition exceeds their normal boundaries.
Annihilus, The Supreme Intelligence, at times the Skrulls and, mostly whenever Lilandra isn't in charge, the Shi'ar emperor/empress tends to fill this role too, especially when Vulcan was in charge.
A large percentage of the giant monsters from Marvel's pre-super hero days were alien invaders (Goom, his son Googam, etc.). Even Groot, who has since done a Heel-Face Turn, started out this way.
A lesser known one is Arm'Chedon(Armeggedon), an enemy of The Hulk from the 90s and leader of the Troyjan Empire. He was so busy conquering planets that he didn't even notice he had two sons running around until one of them got killed harassing the Pantheon(Hulk's team at the time) on Earth, and the other died in trail by combat with the Hulk. He swore vengeance, but got all of one appearance until well over a decade later.
Brainiac of the The DCU. His goals and methods have varied a lot over the years, but he still usually fits this trope.
Mongul (both of them).
Despero and Kanjar Ro are much lower-tier versions of this trope. Kanjar Ro is usually treated as a joke villain these days.
Starro the Conqueror, although his more powerful appearances drift him more towards being an Eldritch Abomination.
Larfleeze of the Orange Lanterns once held an entire solar system under his control, and possesses the power of an entire Corps, though his interests tend more to fulfilling his endless greed. He also commands a vast army of Orange Energy constructs to obey his every whim.
Palpatine from Star Wars is the consummate example of this trope, though he did it through pure Magnificent Bastardry rather than outright force. He got voted into absolute power, after masterminding a civil war after a millenium of peace (in which he was secretly the leader of both sides), resulting in him receiving popular support for his authoritarian reforms. By the time he declared himself Emperor, he already had absolute control of the Republic...or, as he declared it, the first! Galactic! EMPIRE!
"So this is how liberty dies: with thunderous applause."
Ming the Merciless from the Flash Gordon film is referred to as the Emperor of the Universe and the Galaxy. The fact that he can casually cause moons to collide with planets and rain down "natural" disasters on every solar system when he wants to test them definitely places him on the level of Galactic Conqueror.
The Lord Marshal, leader of the Necromongers from the Riddick franchise.
Subverted and played straight with Emperor Zhark in the Thursday Next series. In his book series he regularly destroys and enslaves whole worlds. In person he's a pretty nice guy, though he does tend to overreact a bit, like responding to a shoot out in a Western book by deploying Space Marines.
Paul Atreides, the hero of Dune, brought his dynasty to supreme power by threatening to destroy the all-important Spice.
The Mule, in the Foundation series, damn near scuttles the Seldon Plan, as psychohistory can't predict things like mutants with mind control powers and attitude problems.
The Real goal of Mesa in Honor Harrington, as part of their goals overthrowing the anti-transhuman culture.
Live Action TV
Anubis in Stargate SG-1. As well as the Ori, who have already conquered one galaxy.
Xenu in Scientology conquered many worlds and dropped his excess population into Earth's volcanos, leading to many of the evils that plague mankind.
Traveller: Cleon I, founder of the Third Imperium. In something of a subversion, he not only succeeds, but his empire is actually a more-or-less nice one once it is created - whatever one can say of how he created it. Cleon was a skillful politician, businessman, and diplomat. He knew when to entice, when to bludgeon, and how to weave together multitudinous plans, and in general he was a very Magnificent Bastard.
Albadawi might count. He was a Four-Star Badass who pretty much won the Interstellar Wars himself. In his case he was acting as a servant of the Terran Confederation rather then for his own ambition.
The other "good guys" of 40, the Tau Empire, also have this as a goal. Most of the other races are more interested in some combination of killing, raping, torturing, or just eating everyone. Black and Grey Morality indeed.
Not to mention Abaddon the Despoiler. Warmaster of Chaos. Blessed by all the Chaos Gods. He led thirteen Black Crusades including his 12th to the Gothic Sector against the Imperium (and failed for now). He is now trying to amass all the Chaos armies and unleash his horde on the most prized Imperium worlds, such as Terra, to finally succeed in what Horus didn't.
While each of the Chaos Gods qualify as eldrich abominations, they also struggle against one another for control of the galaxy and the souls and minds of all that live within it in the manner of Galactic Conquerors.
The Zerg Overmind from Starcraft wants to assimilate all life in the universe into the Zerg Swarm.
Later succeeded by his Dragon Kerrigan, who is - by her own words - "pretty much the queen bitch of the universe".
StarCraft II The Dark Voice the true master mind who programed the Overmind to attack the Xel Naga, and lead the Zerg into assimilating the Protoss to create Zerg/Protoss Hybrids. His grand scheme is to wipe out all life in the galaxy so he and his Hybrids will rule it.
Emperor Percival Tachyon, from Ratchet & Clank: Tools of Destruction has managed to bring most of the Polaris Galaxy under his sway
Sargeras the Fallen Titan from the Warcraft universe, whose goal is to lead the Burning Legion to destroy the entire universe.
Mass Effect: The Reapers are an interesting variation of this. In a way, they already control the galaxy, as it is a gigantic coral where they alow humanoid races to develop and return to harvest them every 50,000 years. The entire ancient network of self-repairing hyperspace jumpgates and the gargantuan space station at its center, and most of the highly advanced technology found in ancient ruins was made by them for the sole purpose of creating new galactic civilizations that provide exactly what they need.
Javik is a Well-Intentioned Extremist, mundane version of this. The plan was to get a million protheans to survive the Reaper Harvest by hiding in stasis, then force the primitive races into servitude against the Reapers. Unfortunately, that plan got royally screwed.
The ending to Halo3 just about confirms that the Gravemind and the Flood threat are gone. It consolidated all of its forces on the Ark and the second Installation 4, which Master Chief and the Arbiter have all but destroyed.
Or is it? In Origins II, it seems that the Flood still exist after the battle of Installation 04 (II) but now are too weak to do anything.
The Flood is never too weak to do anything.
BIOS from Forgotten Worlds, who has created gods to serve as his enforcers as he travels the galaxy, conquering and reducing worlds into barren wastelands.
The Grandmaster from Strider and its sequel, who couples it with his desire to create new lifeforms at will to become a new god.
One boss in Visions & Voices is called the "Minor Galactivore." This guy is not even close to being the final boss.
Orbulon from WarioWare tries to be this in the first game. He really sucks at it.
In the firsttwo games of the Kirby Dark Matter Trilogy, Dark Matter threatens to take over, respectively, Dream Land and all of Pop Star. By Kirby 64, it threatens Pop Star and all of the nearby planets.
Lord Krulos of Dino Riders wants nothing better than to return to his star-spanning empire in the future.
In Buzz Lightyear of Star Command, Emperor Zurg successfully conquered the galaxy on several occasions (usually by forcing the Galactic Alliance to surrender to his will, rather than by more "direct" means), but was never able to make it stick for more than an episode or two, thanks primarily to Buzz's interference.
The DC Animated Universe covers many different generations of heroes, across different decades (even centuries) and facing varied and divergent foes, but none can compare to the ultimate Big Bad, whose mere presence is enough to cow raging mobs of bloodthirsty beasts. There is a reason it is his quotation that is the lead-in to the Evil Overlord page.
Cy-Kill from Challenge Of The Go Bots. He's mostly concerned with conquering his homeworld, but he'll conquer Earth as a useful stepping stone toward that goal, and he'll gladly conquer other planets once that's done.
Daemon from ReBoot is a supervirus who can bring entire systems under her control simply by entering them. By the time she actually appears, she has overtaken most of the Net.
Horde Prime from She Ra Princess Of Power; somewhat unusual in that he has in fact already conquered the galaxy in question.
Vilgax in the Ben 10 franchise, who carries the title "conqueror of ten worlds."
Not content with ruling most of the earth, Richard Branson has founded Virgin Galactic, a company which according to The Other Wiki intends to provide sub-orbital spaceflights to the paying public, along with suborbital space science missions and orbital launches of small satellites but come on, we all know what it's really for.
We have space stations, probes have landed on Mars and collected rock samples, there is an American flag on the moon. Scientists are talking about terraforming planets. It won't be long now....