In Robert E. Howard's Conan the Barbarian story "Rogues in the House", "The dust of the gray lotus, from the Swamps of the Dead, beyond the land of Khitai" turns some trapped intruders into this. They have no one to turn on but each other.
Conan himself is a natural berserker. As put in Queen of the Black Coast, "The fighting-madness of his race was upon him, and with a red mist of unreasoning fury wavering before his blazing eyes, he cleft skulls, smashed breasts, severed limbs, ripped out entrails, and littered the deck like a shambles with a ghastly harvest of brains and blood."
Many of Howard's other characters are also born berserkers, including King Kull and Solomon Kane, all the more unnerving with the latter because of his otherwise ironclad self-control, like in "Wings in the Night" when the akaana slaughter his new friends: "Kane laid the body gently down, looking for Kuroba. He saw only a huddled cluster of grisly shapes that sucked and tore at something between them. And Kane went mad." Kane gone mad is frightening as Hell.
In "Black Colossus", The Horde led by Natohk (a.k.a. Thugra Khotan) attacks without thought.
The Lord of the Rings: In the Battle of Pelennor fields, King Theoden of Rohan gets the narrative description: "Fey he seemed, or the battle-fury of his fathers ran like new fire in his veins" Later, when his nephew Éomer finds Theoden's body together with his unconscious sister and believing her to be dead as well, he turns into a full-on Death Seeker. Though unlike the usual version of this trope Éomer is eerily calm about it: straight up reciting poetry in the middle of a pitched battle to urge his troops on.
"Out of doubt, out of dark, to the day's rising I came singing in the sun, sword unsheathing To hope's end I rode, and to heart's breaking Now for wrath, now for ruin and a red nightfall!"
A few characters from The Silmarillion also qualify. For instance:
"Maedhros did deeds of surpassing valour, and the Orcs fled before his face; for since his torment upon Thangorodrim his spirit burned like a white fire within." "Last of all Húrin stood alone. Then he cast aside his shield, and wielded an axe two-handed; and it is sung that the axe smoked in the black blood of the troll-guard of Gothmog until it withered, and each time that he slew Húrin cried 'Aurë entuluva! Day shall come again!' Seventy times he uttered that cry; but they took him at last alive..."
Another case from The Silmarillion is Aegnor son of Finarfin. He has a somewhat tortured persona as a result, because he has a naturally gentle disposition like the rest of his family: but can fly into rages that terrify Morgoth's troops, and his own, in the midst of battle. His name means "sharp flame" referencing his rages.
Beorn in The Hobbit plays this quite literally, being a "skin-changer" who can take on the form of an actual bear in battle. He wins the final battle by going Straight for the Commander while in bear form.
Hurt her husband or her son and Amelia Peabody becomes something far more elemental than an English lady. Watch out for the parasol.
With her grizzly bear morph, Rachel from Animorphs is a particularly apt example of this trope. She's been known to use her own severed arm as a weapon in the heat of battle.
Taur Urgas. He's the Axe-Crazy King of Cthol Murgos, and The Brute of the series' villainous group. He's a total mental case who sleeps in his armour, is prone to fits of madness during which he does everything from commit murder to chewing the furniture, and completely gives into his rage in battle, actually frothing at the mouth as he leads his troops into battle. His madness gives his men a peculiar sense of invincibility, and when he dies (while demanding his enemy come back and fight) their spirits are completely broken.
Barak is a heroic version, who literally turns into a bear and flips out when Garion is in trouble. Bear in mind that Garion pretty much exists solely to get into trouble for the greater good.
But after a while the berserkergang began to come on Valgard, he trembled and frothed and gnawed the rim of his shield, he rushed forward howling and slaying.
Captive Prince: Damen tends to lose himself in battle and belatedly realize how many corpses he's created. He pairs this with Unstoppable Rage when someone threatens Laurent, such as when the clansmen leader feels up Laurent and when the Regent drops the reveal at the Kingsmeet.
Chrysalis (RinoZ): The System has various options for berserking, either as a monster or one of the surface races.
The "Four Blade Berserker" monster automatically goes berserk in combat, as an inherent part of its nature, "increasing its strength but causing it to struggle to tell friend from foe."
Morrelia has a Berserker class that lets her enter a powerful rage while she can maintain her anger, but leaves her fatigued afterward.
Sarah has a darker variant in her Asura heart, feeding on any negative emotion to pump her strength to massive heights, but increasingly robbing her of reason, to the point where she is likely to turn on her allies in a savage frenzy. Once she eventually runs out of targets and calms down, she generally falls unconscious for a while, and doesn't remember everything she did when she awakens — which she finds quite stressful. A big part of her Character Development is coming to terms with her anger and learning to use it without losing herself to it.
The heart fed on her negative emotions, her rage, pain and fear, magnified them, liquified them and sent them pumping throughout her body until every inch of her frame was suffused with them. She could feel it now. The pain of betrayal, the anger of broken trust, the fear of losing herself again. She could feel it all echo through every cell of her body until she was drunk with it.
And it made her strong.
In The Dark Profit Saga, berserkers are members of the Brotherhood of Flame, the most elite of Dwarven warriors. Everyone assumes they "get so angry they go crazy in battle," and it certainly looks like it, especially since they have a hard time remembering the details afterwards. According to Gorm, though, purpose, not anger, is key (although anger has its place too).
Gorm Ingerson: Ye find something in the battle to fight for, something ye'd die for. Your brothers back in the clanhome, the honor of your Da's name, the lives of innocents. A reason to fight, if nothing else, like a tiny fire, and ye reach out and grab it. And ye hold it no matter how it burns. And soon ye can't separate yourself from your purpose, any more than ye could take the light from a candle flame. Ye live to win. Ye can't lose; ye can only die.
Sam Vimes exhibits some of these qualities, especially in the climactic battles of Night Watch Discworld and Thud!. In Night Watch, Vimes taps into his long-buried rage (that he calls "The Beast") and lets it out as he grabs two swords and hacks his way through the enemy, described as "he wasn't an enemy, he was a nemesis." In Thud!, he appears to be doing much the same thing, to the point of ignoring a dwarfish flamethrower being used on him. Although in that case, it was also a case of possession as he was under the influence of a quasi-demonic thing of pure vengeance called the Summoning Dark.
Subverted in Thud! with Mr. A. E. Pessimal - a small man with very shiny shoes who attacks a troll ... with his teeth. It doesn't end well for him.
Among the many, many werewolves are lycanthropes, humans who don't actually shapeshift but maintain a pack mentality and have major rage issues. When the full moon comes around, they go feral and have to hunt something down.
In Side Jobs Gard is explicitly called a berserker during her battle with the Grendelkin. It's all very traditional. She's seen in other battles behaving pragmatically, so it appears it's a choice for her to enter into the state.
A Harry Dresden whose instinctive sense of decency has been outraged is a truly fearsome sight — and the last for anybody or anything that tries to stand against him.
When Harry fully embraced his mantle as Winter Knight he became nearly animalistic in combat. He soon realized that the ability to ignore his wounds and predatory mindset were actually more dangerous to him than his enemy and managed to regain control.
Boïndil from The Dwarves, to the point that he mistook his wife for an orc and killed her while raging. And he's a good guy.
Adus of The Elenium becomes this towards the end. Already an Axe-Crazy mentally-handicapped Psycho for Hire, he loses his mind during the climax, to the point where he cuts his way through his own troops to get at Kalten.
Logen Ninefingers from Joe Abercrombie's The First Law series is a pretty tough fella normally, but when he's desperate and driven to extremes, he completely loses control and becomes "The Bloody-Nine," an unstoppable killing machine who holds no distinction between "friend" and "foe," only "dead" and "soon to be dead." Logen hates this Superpowered Evil Side of himself and spends a good portion of his life trying to escape it.
Wulfgar, the barbarian from the popular Drizzt novels, seems to fit into this type in almost every fight he's in after being resurrected after a decade of torment by Errtu. Unable to cope with the hopelessness and torture he endured, and fearing that his escape is all some dream, he fights recklessly. Several times, Drizzt and others have had to divert their tactics to save him from himself.
The dwarven Battleragers, who love combat and jump into it with a glee that scares their allies as much as the enemy. Considering that their fighting style incorporates armour that can only be described as a mobile cheese grater which is used to shred opponents by rubbing against them with furiously, this is probably justified.
In the Starlight and Shadows trilogy, Fyodor is a berserker from the land of Rashemen, which is well known for its warriors being able to enter a magical berserker state. Fyodor, however, is unable to control his berserker rages, and is exiled from his homeland due to the danger he poses to his allies.
In The Generalist you have both Frank and Dash, through different means. In Frank's case, if he gets pushed too far beyond his limits of control, or if his rage takes over, or if his quirk of nature "Overkill" gains control of his body, then he can fly into an utter Berserker rage. Dash, on the other hand, has a far greater chance - each time he uses the ability Maximum Troll, his chance of slipping into a Monster-gene fueled berserker fury rises.
In Taboos 2 and 3, he actually does lose it, forcing Frank to hit him with the Roadbuster. The first two times broke him out of Maximum Troll, the third was just for fun.
Harry Harrison and Tom Shippey's The Hammer and the Cross trilogy features a realistic Nordic berserkr as a major supporting character. When not fighting, he's brooding and melancholy, prone to fits of heavy drinking. When fighting, he's a Death Seeker. One of the main characters notes that all "true" berserkrs are inherently Death Seekers.
In A Harvest of War Wild Rhona claims to be able to enter a berserkergäng without drugs. She doesn't seem to need it much, though.
Grigoriy Pechorin, the Byronic Hero of Mikhail Lermontov's A Hero of Our Time, has extreme ennui for a fatal flaw and so lives by this principle as well. He leads charges on enemy positions, enters a duel he knows to be rigged and volunteers to tackle an Axe-Crazy drunken Cossack.
In the world of The Icelandic Sagas, a warrior in berserk-fury is not only supernaturally strong and ferocious, but may also be invulnerable to fire and regular weapons. The sagas' outlook on berserkers and berserking is very ambiguous, because the older pagan tradition saw these behavior or ability as a good thing, while the Christian Middle Ages viewed it either as mere superstition or as evil pagan magic. As a result, when berserkers (berserkir) appear in the sagas, they often do not actually fit this trope: Often, they are merely bandits and troublemakers who live by robbing and blackmailing people and who never actually prove the supernatural ferocity that they claim to possess; the implication is that they just want to scare people into submission with their bogus "berserk powers". Consequently, the sagas will usually avoid calling (especially heroic) characters that display actual berserking behavior "berserkers", presumably because the term is so strongly connected to the aforementioned villain stereotype. Some examples of saga characters that go on berserk rampages without ever being called "berserkers":
Kveldulf, Skallagrim and especially Egil of Saga of Egil Skallagrimsson frequently fall into berserk-fury in combat.
The Saga of Hrolf Kraki: The hero Bodvar Bjarki goes on a furious killing rampage in King Hrolf's final battle.
The Saga of the Jomsvikings: Aslak Holmskalle and Vagn are the first to board Jarl Erik's ship and "each plowed forward on either side of the ship, clearing the deck, so that everybody fell back". They "[slay] many a man". Aslak is moreover invulnerable to weapons and though he wears no helmet, and gets hit on the head with swords, he stays unharmed. Aslak is only stopped when his skull is bashed in with an anvil. Vagn, who lacks invulnerability but is nevertheless "killing men savagely", continues to advance until he takes a hit with an oaken club which splits his helmet and prompts him to jump back onto his own ship. Jarl Erik then retreats from the battle line, because Aslak and Vagn have killed too many defenders.
The Saga of the Sworn Brothers: In the Battle of Stiklestad, Thormod Kolbrunarskald (who has already made clear that he intends not to survive King Olaf) always fights in the front line without armour or shield, instead wielding a large axe with both hands, with which "he hack[s] his way through the ranks of the enemy". His behavior earns him "great acclaim for his bravery". Nevertheless he is still unwounded when King Olaf has fallen, much to his own regret. Flateyjarbók furthermore adds that Thormod folds up his tunic under his belt in the front, but lets it hang low in the back because he intends to only go forward and never retreat; that "so many men fell by Thormod's hand that naming them all would take far too long"; and that he stays unwounded because the enemies are scared to face him.
Incarnations of Immortality: In Wielding a Red Sword, Prince Mym fights like this when he tastes blood. Thanks to his incredible training and discipline, he retains his fighting skills and can direct his rage at his true opponents.
Galbatorix creates groups of magically modified soldiers who cannot feel pain in the third book. They disregard their safety because they can take crippling injuries and continue on, making them a whole army of berserkers.
Roran Stronghammer once killed nearly 200 men in a single battle and when he starts fighting is said to experience a battle rage that lends him strength and allows him to overcome many obstacles. Though he also still relies on smarts and cunning.
The Iron Tower has Danner Bramblethorn, a Warrow berserker. During the last battle of the trilogy he charges all of the Goblins on the walls, ranting about how he is "King of the Rillrock".
In The Island of Sheep, Richard Hannay assists Valdemar Haraldsen, a Danish scholar being pursued by treasure-hunters seeking to get their hands on his father's fortune. During the final confrontation, which takes place on Haraldsen's home turf, the eponymous island, the usually-mild-mannered Haraldsen goes berserkr and single-handedly captures the Big Bad and throws him off a cliff. Afterward he immediately faints, and has no memory of the incident when he wakes.
Knaves on Waves has Captain Carnage and his crew, who absolutely delight in battle. This makes them terrible negotiators, a to the point that they recruit others to do their talking for them.
Exploited Trope in Frans Bengtsson's The Long Ships, where seven vikings manage to take a ship and scatter some 50 enemies because they think the seven look like they might be berserkers. Of course, if seven unarmoured men attack fifty heavily armoured, the assumption isn't unreasonable...
The Koloss from Mistborn are a race of berserkers. They're used as shock troops by The Empire and were in fact originally created for this purpose, but in the 1st book the protagonists don't have to worry about them since they're operating La Résistance in The Empire's capital, and the koloss's blood rage means they can't ever be allowed near major population centers you want to leave standing. In the 2nd and 3rd books after the Lord Ruler, who was essentially restraining them telepathically, dies, the koloss start wandering around in hordes and killing anyone they can get their hands on. Eventually the new Big Bad, Ruin, takes command of them.
Touchstone goes into rages that give him frightening power and disregard for things such as physical impossibilities (i.e. incredibly powerful feats of magic, carrying someone weighing about as much as him and running full tilt up a hill, trying to hoist up a throne affixed to the floor so he can throw it at someone). He regrets these bitterly. Being a Berserker seems to run in the Royal bloodline, so he would have inherited it from his mother, though whether or not she or any of his half-siblings were Berserkers is not specified. It is revealed towards the end of Sabriel that this is partly why he was frozen as the figurehead of a ship for 200 years.
His son, Sam, inherits this trait to a degree. Perhaps due to his more cautious disposition, he never loses himself to the extent that Touchstone does.
The title character of the prequel Clariel also has this as does her mother, Jaciel. They both have Royal blood, though are only cousins to the current monarch, and her rages are explored in more depth; she tries to use various techniques learned from a book on berserkers to try and keep a lid on it, but the wild, unrestrained passion of the berserker rage also gives her a natural affinity for Free Magicthat will, eventually, turn her into Chlorr of the Mask.
John Ringo has pretty much every Keldara (except for Lasko and Kacey, but specifically including the Kildar) turn berserker at the end of Unto The Breach. There is even a discussion of the various states of combat capability based on heart rate, blood pressure, etc.
Skandians from Ranger's Apprentice series can (unintentionally) enter sort of a berserker state, when they gain super-strength and supernatural resistance to pain and wound. And become Ax-Crazy. Most of them doesn't survive the experience, though.
Robin Hobb's Realm of the Elderlings: Fitz, protagonist of the Farseer and Tawny Man trilogies, tends to go into a "battle haze" whenever he fights, disregarding his own safety to savagely lay about himself. This makes him very effective during battle, but then he's left standing around delirious for a few minutes until the berserker-mode wears off.
The Badger Lords, and anyone else unfortunate enough to have the bloodwrath, from Brian Jacques' Redwall series. When the bloodwrath takes over, the warrior will throw themselves into battle, seeking to reach and kill their mortal enemy (their eyes glow red, even in darkness, and one point-of-view shows that everything around them kind of disappears into red mist with only their archenemy appearing clearly). They are completely heedless of their own safety, and will kill anyone — friend or foe — who tries to get in their way or otherwise stop them. They usually end up killing scores of foes, their mortal enemy, and themselves.
The Rifter: John when his Rifter self is unleashed. He becomes a divinely-powered whirlwind of fury, smashing everyone and everything in his path; he charges straight into armies, taking innumerable wounds, whose pain fuels his rage and which heal instantly. He is almost as dangerous to friends as to enemies, especially when he shatters buildings and creates gaping chasms in the earth.
Second Apocalypse: Cnaiur urs Skiotha is an extremely unhinged warrior of the steppes. As an Armoured Closet Gay in a macho culture who harbors a deep shame in his past, he's a raging ball of insecurity and lunatic ferocity in a desperate attempt to cope. He's prone to Unstoppable Rages, bellowing things like, "I am Cnaiur urs Skiotha, most violent of men! Who will murder me?!"
Jarek of The Seventh Tower. Milla almost turns into one when Tal sells her shadow in Aenir but this trope is defied when she uses meditative breathing exercises to calm herself down.
Erik Hakkonsen from The Shadow of the Lion series is capable of this, but since he doesn't know friend from foe once he enters a rage, he refuses to do it when the prince he's guarding is nearby.
Star Wars Legends: In Wraith Squadron, the multiminded alien character "Runt" had a "pilot mind" who was a berserker. As berserkers make very bad pilots, Runt did badly enough to qualify for the Wraiths. With his wingman's help Runt eventually got over this problem.
According to Audie Murphy's autobiography To Hell and Back, there were two occasions when he started firing an oversized gun, cursing, and giving no regard to his own safety. He says that the memories of these events are very fuzzy, like it was a dream.
In Gregor and the Prophecy of Bane, Gregor learns he's a "rager" a person with highly developed warrior skills, a natural-born killer. Meaning he has a natural capacity to go into this state when his life is in danger. It's not always voluntary, which is really bad for a character who normally hates to kill.
Ripred is also a "rager".
Waltharius: In Attila's service, Walther leads a Hunnish army against a rebellious tribe, and "rages in the midst of the battle" (v. 196), "mowing down everything in his way" (v. 197). The enemies are so terrified of him that they flee from him "as if they had seen death" (v. 199).
David Weber's The War Gods series features a species of these, the hradani. It's somewhat involuntary, and they're not happy about that.
In James Swallow's Warhammer 40,000 novels Deus Encarmine'' and ''Deus Sanguinius, the Blood Angels are perpetually tempted by their "flaw", the "red thirst", which transforms them into this when they succumb. Stele unleashes it in opponents to be rid of them. At the climax, Rafen succumbs to this; on the other hand, it does unlock the powers of the Spear of Telesto for him, and the daemon he fights is shocked to see that the many futures in which Rafen failed instantly vanish. Then the spear protects him. When the dying daemon unleashes it in the other Blood Angels, they terrify their enemies, who retreat although they never retreat, and the spear even, astoundingly enough, lets Rafen bring back his battle brothers who had succumbed.
Derfel muses several times about how any man, including a justice loving generous soul like Arthur or a loving family man like Derfel can transform into a monster in the heat of battle, especially when victory seems likely.
A terrible hate wells up in battle, a hatred that comes from the dark soul to fill a man with fierce and bloody anger. I knew that Saxon shield wall would break. I knew it long before I attacked it. The wall was too thin, had been too hurried in the making, and was too nervous, and so I broke out of our front rank and shouted my hate at the enemy. At that moment all I wanted to do was kill... so I ran ahead, madness filling my soul and exultation giving me a terrible power as I picked my victims. They were two young men, both smaller than me, both nervous, both with skimpy beards, and both were shrinking away even before I hit them. They saw a British warlord in splendor, I saw two dead Saxons.
There's also his description of the Irish Blackshields, a renegade army of berserker soldiers and raiders.
The Blackshields did not attack in a line, but came in a howling mass. This was the Irish way of war, a terrifying assault of maddened men who came to the slaughter like lovers.
In Robert Low's The Whale Road one of the Viking ship's crew is a skinny man with a bad leg named Pintel. Throughout the story the main character, Orm wonders why no-one ever mocks Pintel over anything and why the man is even able to mock their Captain while anyone else is threatened with death. Later on a newer Christian member of the crew pulls down Pintel's offering to Odin and in the ensuing argument mocks Pintel's leg. Pintel challenges the man to a fight. At the beginning of the duel Pintel throws away his shield and begins to froth at the mouth. Pintel then leaps onto the challenger and hacks at him until there is very little left other than a lot of blood and some blocks of flesh.
Who's Afraid of Beowulf? features Starkad the Berserker, the sweetest, kindest if rather dim hero you'd ever want to meet — outside of battle that is.
The Witchlands has an understated version of berserkerism called the Nihar rage, which affects members of the Nihar family. They're extremely Hot-Blooded, cursed with Hair-Trigger Temper, and more prone to physically attacking someone. When Vivia feels it for the first time, she notes that it makes her fearless, focused and gives a boost to her witchery, and Merik implies that there's some low-level magic to it.
