Saiyans are stated to grow stronger when they are beaten to near death, and Vegeta uses this to his advantage on Namek by having Dende heal him after intentionally letting Krillin almost kill him.
Later, Cell self destructs and reforms, causing this to happen because he has Saiyan cells. This is the last time this Saiyan trait is specifically mentioned.
There's also the example of dying and then training in the afterlife, especially with King Kai. Goku learned the Kaio Ken, Spirit Bomb (AKA the Genki Dama), and gained his Super Saiyan 2 and 3 forms this way.
There's also Freeza, who is maimed and near death when his shattered body is discovered by his father and rebuilt with cybernetics to be even stronger than before. It doesn't help.
In Zatch Bell!, Kiyomaru dies or comes very close to it in Faudo, and when he reawakens his heart power is massively increased and he gained the answer talker ability.
The best way to increase one's spirit energy in Shaman King is to die, severly injured, or be in a near death state because this brings one closer to the spirit world. All of the main warrior characters die at least once in the manga, and some die several times. In the anime, the Tome of The Shaman enabled the main characters spirit energy to raise, and after Len and Yoh hadNotQuiteDead nearly been killed they came back stronger, with Len curbstomping 1/3 of Zeke's followers combined with The Power of Friendship, and Yoh was able to give his brother a Villainous Breakdown.
In Aflame Inferno, the main character gets possessed by Inferno after his nearly fatal blow.
Kazuki of Busou Renkin gains his renkin as his heart because he almost dies. He also pulls this trope again in episode 14.
Yusuke from YuYu Hakusho did not awaken his spirit powers until after his death, and later he awakened his demon powers after his second death. It's outright stated that the latter instance happened because of the former. At the time of his first death, Yusuke was merely a slightly strong human. At his second, he was an A-Class fighter who could send someone flying with the shockwave of a relatively weak punch.
Phoenix Ikki in Saint Seiya. Every time he's killed, he comes back twice as strong. Given that he died quite a few times, he's a undisputed powerhouse now.
In Bleach, the straightest example would be Ichigo's near death, or hollow, transformation to regain his spirit powers. The normal example of dying and becoming a shinigami or a hollow does not work because you never come back to life, you stay dead. Ichigo's Hollow form is a unique example. Whenever Ichigo is Only Mostly Dead or outclassed and exhausted it tends to take advantage of his weakness by taking control of his body.
After Ulquiorra blasts a hole straight through Ichigo's chest, the Hollow's powers go completely insane (seemingly neitherIchigo nor his Inner Hollow, which while vicious is also quite coherent, is in control, and instead their combined powers form a being of pure instinct), and tears apart Ulqiuorra and nearly kills Ishida. It also tanks everything thrown at it and is only stopped by one of its own attacks
Brook from One Piece plays this somewhat straight. He died and came back as a skeleton, so, while he isn't explicitly stronger, he is much faster now, a lot harder to kill, and able to do things like run on water or jump distances that are ridiculous even by One Piece standards. This is all explained by skeleton!Brook apparently having the same level of strength that human!Brook did, but vastly lower mass.
In Fullmetal Alchemist, Ed and Al both nearly die when they try to resurrect their mother. They barely survive and gain the ability to use alchemy without circles, and Al is also a Made of IndestructiumAnimated Armor. Overlaps with Blessed with Suck, since Ed lost a leg and an arm in the process, and Al loses his entire body, and cannot eat, sleep, or experience any senses aside from seeing and hearing.
Code:Breaker's Ogami pulls this trope twice. The first time when he was a child and received the Emperor's flame, and the second when Yuuki kills him and the Emperor allows him access to the next flame for making the correctchoice.
In King of Thorn, Marco Owen is killed by Zeus. However, another character sacrifices her life to bring him back, and the new Medusa-enhanced body she gives him has useful new abilities such as a resistance to Zeus's electrical attacks.
Over on Naruto, you could technically apply this to anyone brought back through Edo Tensei / Reanimation, since the zombies it creates can regenerate from ash and have unlimited stamina. Too bad you still have to follow the summoner's orders unless you're Itachi or Madara. The stand-out example would be (the REAL) Madara. He's physically in his prime but has allthe modifications he could only make after he was too decrepit to show them off, and doesn't have to worry about little things like being bisected.
Surprisingly Madara came back even stronger after being truly resurrected through Rinne Tensei. Apparently, having a real body gives him access to techniques enough to curb-stomp the 9 bijuus, Naruto, Sasuke and Tobirama altogether.
The said curb-stomp resulted in Naruto and Sasuke having a near-death experience. They came back, with Rikudo Sennin's powers, namely Rikudo Senjutsu and Rinnegan, power-ups enough to match the now Jichuuriki Madara.
Tenchi and Sasami end up with this thanks to Tsunami in the Tenchi Muyo! OVA. Tsunami bonds with Sasami and Tsunami ends up rescuing Tenchi from death after a counterattack ended up backfiring. Now Sasami's stated to become Tsunami in the future and Tenchi is now the avatar of that universe's God.
In Puella Magi Madoka Magica The Movie: Rebellion, Sayaka Miki comes back to life as a master swordswoman, able to pull off tricks like stabbing Homura's time shield before she can use it to freeze time. It's explained that she gained the memories of her past incarnations from the "Groundhog Day" Loop, giving her tremendous experience. She also gained the ability to summon her witch form, Oktavia von Seckendorff, to assist her in battle.
Battler, from Umineko: When They Cry, takes Dlanor's Red Key through the chest at the climax of Ep5. His death gives him as much time as he needed to think back over the mysteries of the novel, realise what he'd been missing, come to understand the Truth of the Game and will himself back to life as one of the most Bad Ass and powerful characters in a series filled with already-overpowered characters.
Ryougi Shiki in Kara no Kyoukai gains a stronger version of the same ability when one of her two personalities dies, leaving a hole to "Origin" in its place. Effectively her new second personality is the universe itself, and possesses unlimited power.
Yu-Gi-Oh!: According to the Gagaga Academy Tospedia card storylines, "Reaper" was on the verge of death due to the out-of-control magic he was using. "Hierophant" and "Empress", unwilling to stand aside any more, were determined to try and save his life by exorcising the Dark Magic inside him by amplifying the Light Magic that existed inside him all along. At the end of the intense conflict between Holy and Wicked magic, he ended up being revived in a miracle.
In the game itself, there are cards that trigger Came Back Strong in both players and creatures. Tuktuk the Explorer does this to himself: A 2/2 for three mana, he's beneath the curve. When he dies, you put a 5/5 token into play called Tuktuk the Returned.
The new "Undying" rule leads to creatures that have undying responding to being Doom Bladed by returning from the graveyard with slightly higher power and toughness. Hilariously, if combined with persist (which leads to "came back weaker"), you can get creatures that simply never die.
Watchmen's Dr. Manhattan also fulfills this trope, although it's not solely his death that is the catalyst of his powers, but the way he died. That is being disintegrated at the sub-atomic level but remaining conscious and disembodied for months before figuring out how to make a new body.
In the original Thanos stories in the 1970s, this was how Adam Warlock "finally" defeated Thanos.
In Invincible, Allen the Alien becomes stronger every time he regenerates from life-threatening wounds. At the beginning of the series, he was barely able to fight the Virtumites one on one. Now he's more than a match for one or two of them. With the exception of Grand RegentThragg.
This temporarily happened to Doctor Doom in Mark Millar's Fantastic Four run. He gets thrown to the past by a villain called the Marquis, and survives as he put it "by sheer hatred", Pre Retcon that is. Dark Avengers shows readers that he's saved by the time traveling Thunderbolts. It still doesn't explain his supposed power-up being lost, though his power up was regarded as Canon Discontinuity by all the other writers since.
Later, in the Planet X arc of New X-Men, Jean and Wolverine are headed into the sun, and Wolvie kills Jean to spare her the agony of incineration. She soon awakens in full Phoenix mode, reshapes the asteroid into a ship with the power of her mind, and easily returns to Earth. And this time, she came back right.
In the Star WarsLegacy comics, Big Bad Darth Krayt comes back from the dead and is no longer bothered by the crippling Yuuzhan Vong implants while also becoming more formidable and Drunk on the Dark Side than ever. He then attempts to invoke this trope on Cade Skywalker, hoping Cade will have the same reaction with The Dark Side and finally be his disciple, but it just encourages Cade to embrace the Light Side and kill Krayt.
Doctor Strange, as part of his trials to become Sorcerer Supreme, had to meet Death in combat. When he realized he couldn't defeat or escape Death, he surrendered entirely to it, died, and was restored to life—now functionally immortal.
This happens to Sing in Kung Fu Hustle. Beaten practically to death, he somehow emerges from his cocoon of bandages as a superpowered master. The beating apparently opened all the previously blocked chi paths in his body, unlocking his heavily-foreshadowed hidden potential.
Star Wars's Obi-Wan claims that if Vader strikes him down, he'll become even more powerful. Obi-Wan's spirit makes a few appearances in the rest of the series, but whatever power he does have seems limited to influence. He's probably referring to merging with the Force.
Charlie St Cloud: After almost dying in a car accident, Charlie gains the ability to see and speak with the dead.
Resurrection (1980): After surviving a car crash that killed her husband, a woman finds that she has miraculous healing powers.
Sara of The Craft nearly died but she managed to invoke the power of Manon.
Happens twice to King Ghidorah in the film GMK. Once when Mothra sacrifices herself and Ghidorah absorbs her essence to become stronger and once when Ghidorah absorbs an artifact from the shrine in which he was placed/worshiped. However, it's still not enough to defeat Godzilla.
Occurs to Godzilla himself in Godzilla VS Mechagodzilla II when Rodan sacrifices himself to revive Godzilla after Mechagodzilla nearly kills him. Godzilla becomes more powerful because of it and gains his most powerful attack-the Spiral Beam, which he uses to utterly obliterate Mechagodzilla.
Neo of The Matrix only gets to awaken his spoon-bending powers after being killed by Agent Smith in the first movie.
Alex Murphy is killed in action in RoboCop (1987), but he is reconstructed as the titular cyborg, an implacable crime-fighting machine.
A premature death (usually violent) is what awakens the quickening of the immortals of Highlander which enables them to live forever and rapidly heal from any injury.
Jason from the Friday the 13th films. In his first appearance (which happened in the second film; his mother was the killer in the first movie) he's just a regular human. A very large, strong, and tough human, but still a human. But after he's killed he comes back as a near-unstoppable undead abomination.
In Scott Pilgrim vs. The World, Scott gets killed by Gideon, gets trapped in limbo, comes back (because of his extra life). He manages to reconcile with Kim, apologize to both Ramona and Knives for cheating on both of them, and then finally kicks Gideon's ass.
In The Lego Movie, Emmett becomes a Master Builder after "dying", getting a glimpse of the human world, and returning.
In Dune, Paul Atreides almost dies when he drinks the water of life, and when he wakes up he is the Kwisatz Haderach.
In The Lord of the Rings, Gandalf the Grey dies and comes back as Gandalf the White with augmented powers and authority. He is a lesser-angel figure on a mission from greater-angels and hasn't completed his quest so he is sent back and improved.
Stephen King's The Dead Zone: A man is critically injured in a car accident and is in a coma for five years. When he wakes up, he has psychic powers, including precognition and psychometry.
In Eoin Colfer's The Supernaturalist, people who've undergone a near-death experience can see the Parasites - not always permanently, but that's how it is for all the named characters - and are therefore able to fight them. (So can Bartoli babies, but that's beside the point.)
In Grave Peril, at the climax of the book Harry is killed by the Nightmare in his dreams, and is immediately revived via CPR. When he comes back, Harry has created a ghost of himself (ghosts being psychic copies of the individual who died and are created at the moment of death) and the two proceed to whup ass. While doing so, Harry eats the Nightmare in his dreams and thus absorbs his magical power, allowing him to come back with so much strength that he vaporizes a pair of vampires almost instantly with no effort.
To some extent when he was assassinated—he was already the Winter Knight, but he was incredibly drained and had nothing material left—not his house, his car, his weapons, his pets, his equipment, only a single change of clothes... and then he spent most of a book as a powerless ghost. Now? The Winter Knight is back in town, and just dictated terms to Queen Mab.
A variation happens in the Dragaera novel Issola when Lady Teldra gets killed with a Morganti weapon. Vlad uses it to create a new Great Weapon, which makes him permanently stronger, starting with slaying a Jenoine.
There's a short story where colonists on an alien planet deal with escalating threats from local fauna, starting with pests that feed on their crops and working up to large predators. Eventually someone works out that it's the same life form, coming back stronger each time they wipe it out. He realizes this just in time to stop them from killing an alien that looks human:
"I think we can deal with this one. What I don't want to face is what comes next."
In Harry Potter and the Deathly Hallows, a variation takes place: Voldemort uses Avada Kedavra on Harry, technically killing him. But since Harry is an unintentional Horcrux, all this does is kill the bit of Voldy's soul inside Harry, so he gets better and comes Back from the Dead. Cue Voldy wanting to humiliate him by throwing the body around using the Cruciatus Curse and Harry being completely immune to the pain he expected to come with the spell. However, that last part is actually due to Voldemort's use of the Elder Wand, which refused to harm Harry, its true master.
In the world of The Belgariad, resurrection in any form was impossible until the Orb Of Aldur was recovered.
When Durnik, (aka 'The Man with Two Lives'), was killed by Zedar, the Gods conferred and agreed unanimously to allow his resurrection by Belgarion, Errand and the Orb at Polgara's pleading. He went from being an ordinary man, to being an immortal Sorcerer later proven capable of slam-dunking a DEMON LORD singlehandedly.
Earlier in the series, Belgarion brings back a colt that died immediately after birth. The horse the colt grows into is supernaturally strong, able to translocate, apparently highly intelligent (well, for a horse, anyway), and quite probably immortal. Fitting, as he turns out to be fated to be the horse of a god. Belgareth and Polgara conclude that the only reason that he succeeded was that he didn't know it was impossible. Well, that and doing it in a cave the Gods had used as a meeting place before they left the world.
Trapped on Draconica: Dragokin Erowin can handle mooks. After dying and becoming an angel she can effortlessly No Sell the Big Bad's best shot.
This is a belief of the followers of the Drowned God in A Song of Ice and Fire. "What is dead can never die, but rises again, harder and stronger." They invoke this in a baptism ceremony in which believers are "drowned" and revived through CPR. The most devout believers do this multiple times.
When Xander resuscitates Buffy from her death at the Master's hands, she comes back stronger, knows instinctively where he's going, and is no longer vulnerable to his hypnosis.
She also seems to be stronger after her second death, given the way she pwned the leader of that demon gang (who tore off a vampire's head with his bare hands!).
This is what happens in Kamen Rider 555 when someone is killed and revived as an Orphnoch.
Kamen Rider Agito introduces Gills, who died of his powers breaking down his body but was revived by a psychic with healing abilities. The result not only cures him of his degrading body but grants him the power to become Exceed Gills.
Gentaro Kisaragi, a.k.a. Kamen Rider Fourze, is killed in #31 by Kamen Rider Meteor (who made a Deal with the Devil with the Ares Zodiarts to save his best friend), only for Kengo to use the Cosmic Switch (which they hadn't been able to get to work before that) to revive him, giving him his Super Mode in the process.
Phoenix, one of the Greater Phantoms is able to revive himself every time he is killed, just like the mythological bird he takes his name from. According to him, every time he is revived, he gets stronger.
Subsequent episodes bear this claim out. He not only become immune to the same attack that killed him previously it takes more attacks of a similar level to take him out the next time, more resistant to all attacks in general and the time he takes to regenerate and recover become shorter and shorter each time. This ability ultimately means Haruto can't kill him and has to turn him into Sealed Evil in a Can by trapping him in the sun.
Happens later with Dragon, Haruto's Inner Phantom and the source of his magic. The Legion Phantom enters Haruto's underworld and ends up destroying him in the resulting fight. Eventually, Dragon is brought Back from the Dead by Haruto's absolute refusal to give up hope no matter how heavily the odds are against him. This results in Dragon being able to grant Haruto access to Infinity Style and gives Dragon the power turn into the Axcalibur sword.
In Charmed, when Cole arrives in the underworld, he finds that the souls of demons have their powers eaten by the creatures that live there. He decides to get in on that action, and collects enough powers to return to the land of the living, and with almost every demonic power there is. Unfortunately, it eventually drove him crazy, and he wanted to die, but couldn't.
In Norse Mythology, Odin sacrificed himself on Yggdrasil and after he came back to life, he knew magic.
Sedna is just an ordinary woman until her father chops off her fingers and throws her into the ocean. She becomes the goddess of the ocean, the most important goddess of the Inuit cosmology because it's only with her on their side that the people can avoid starvation. Her fingers turn into seals.
Another version has her being thrown in the ocean first, followed by the chopping off of her fingers (which turn into seals), hands (which turn into walruses) and finally her arms (which turn into whales) to get her to stop clinging on to the boat.
In Yoruban mythology and Santeria, Shango. He was an ordinary king until he hanged himself and became one of the most powerful (and popular) Orisha. His salute means "the king is not hanged".
When the first season of NXT started, fan favorite Daniel Bryan lost virtually every match he was in the entire time. His heart and determination kept him over with fans, but he just couldn't break the glass ceiling of actually winning a match. After getting fired, he came back weeks later to help team WWE take on the Nexus. Daniel Bryan immediately made one of his opponents tap out, and went on to play a major role in the match, as well as future WWE matches and storylines. Over the years, he's built himself up into a heavy fan favorite to the point of not only becoming one of the new top guys in WWE, but also main eventing Wrestlemania 30 for the World Title and winning!
In Brave New World characters gain super powers by undergoing a near-death experience while in mortal danger.
In the Pathfinder tabletop game, being born dead and then coming back to life is one of the possible origins of a sorcerers powers.
Geist The Sin Eaters: As mortals, Sin-Eaters typically have some sort of connection to fate or the world of the dead (represented by glimpses of ghosts or strange hunches). Once they die, however, a geist offers them the chance to come back, and when they accept, they gain access to a whole suite of powers.
In order to become an AbyssalExalted, you have to be on the exact verge of death at the time.
In Dungeons & Dragons, this is how the Risen Martyr Prestige Class works. A saintly character is temporarily resurrected after their martyrdom and given extra powers to complete their unfinished task... in theory. In practice the class is terrible note The "Iron Chef" competition, centered around making builds that allow even the worst classes to be playable, refuses to run it because is THAT bad, doesn't give you the choice to pick levels in anything else, and the final ability removes you from play again. Even in the worlds without resurrection that it is intended for, it's more practical to just make a new character.
The Sisters of Battle have their Holy Living Saints, revered fallen martyrs who return to the battlefield as miraculous glowing giant warrior of faith when called by their pious sisters to fight in the Holy God Emperor of All Mankind's name.
The Space Marines have their Dreadnaughts. A doomed Space Marine, if deemed suitable, can be inserted/sealed in a "Combat Sarcophagus" which makes the Space Marine even deadlier in battle.
Chuubo's Marvelous Wish-Granting Engine: in the Glass-Maker's Dragon campaign, playing "The Other One" - i.e. an OC rather than modifying one of the pregenerated characters - involves dying quite early on and coming back on a Miraculous Arc.
Zettai Hero Project: Unlosing Ranger vs. Darkdeath Evilman has this as one of the core mechanics. Every time you get killed, you lose the items you're carrying and get sent back to level 1, but your basic stats increased depending on your "total level earned", and stats boost you get from level up depends on your basic stats. As Pirohiko says, a hero always comes back from the brink of defeat to save the day!
Mega Man X's Zero died the first game, was rebuilt in the interim, and came back with upgraded armor and weapons in the second game. A lesser version occurred with X5 and X6. He didn't get new armor, and the offensive boost was a tweaked Arm Cannon, but he did gain a Double Jump as a standard ability.
Used in the Disgaea games. You level up, then you reincarnate into a new body that starts with better stats. The game more plays with this trope.
Sonic the Hedgehog (2006) features the controversial kiss from Princess Elise that not only revives Sonic from dead-but-not-really-death but transforms him into Super Sonic for the final boss.
Second generation Death Knights combine this trope with a bit of Came Back Wrong. Most Death Knights are already powerful Warriors or Paladins to begin with, but when they are raised they are imbued with powers over necromancy and disease, making them the Scourge's most powerful soldiers. This is less evident in World of Warcraft due to the obvious balance issues making Death Knights more powerful than the other classes would cause, but from a lore standpoint it remains true, even though it only translates into a higher starting level.
In World of Warcraft Cataclysm's Rage of the Firelands patch, you confront Alysra and defeat her single-handedly in a quest. After you do so, some Druids of the Flame arrive, and revive her as a fire hawk, resulting in her flying off to the Firelands and becoming a much more difficult raid boss that requires 10-25 players to defeat.
Ingvar the Plunderer in Utgarde Keep combines this with Trick Boss. After he's reanimated as an undead, his abilities become more powerful. Similarly, the Black Knight goes from being defeated alone in a quest to being a 5-man dungeon boss in Trial of the Champion. The Scourge had a habit of handing out this type of upgrade during their heyday.
Happens to the Big Bad of Final Fantasy II. The Emperor comes back to life as a lich king, having taken control of Hell and its legions. In Soul of Rebirth, his good side took over Heaven.
In Final Fantasy IV this happens to a few bosses. When you kill the Earth Fiend Scarmiglione, he comes back as a zombie, attacking from behind or Dr. Lugae, he comes back as a cyborg.
Before the events of Tsukihime, Tohno Shiki has the ability to see "things which are hidden". When he is killed and brought back to life, this mutates into the ability to see death, allowing him to cut anything by tracing its "lines of death" or destroy anything by stabbing its "point of death". This manages to be even more overpowered than it sounds, but Shiki has to wear Anti-Magic glasses to prevent going mad (besides the obvious reasons, just looking at a "very immortal" character for too long can cause brain damage).
The Nameless One in Planescape: Torment sometimes regains memories after he dies and comes back, which can translate into more experience points, and hence more power.
In Starcraft II, one mission against a Hybrid Monster named Maar has him repeatedly assaulting your small base. If you kill him, he keeps coming back, larger, with more hit points and a more powerful attack.
Additionally, Commander Gore is killed very early into the mission. However, the forces of the Schwarzwelt bring him back as a puppet and an observer, in a type of zombie called Ubergestalt. When he shakes off control, he reveals that not only has he regained his humanity, but he now possesses supernatural abilities and superhuman traits. Not only does he go from a Sacrificial Lion to one of the most pain in the ass bosses in the game (in the routes you fight him on), but he also demonstrates the ability to see potential futures.
In Ogre Battle series (and Tactics Ogre games from the same universe) it's possible to resurrect your fallen units as undead. They have a weakness against enemy clerics, but are often much stronger then they was in live. In Knight of Lodis it's possible to resurrect fallen knights as Angel Knights, one of the best classes in game, who doesn't have weakness to holy.
Red Savarin from Solatorobo says he feels better than ever after his brush with death. It eventually results in him being able to Trance and become extra-powerful when he feels the need to protect someone.
Asura from Asura's Wrath, after dying once, comes back and can now utilize his Six Armed form (Which at one point was only usable with his daughter Mithra's Help) as well as newer, more powerful forms after it.
In Wizard101 when Malistaire is brought back as a Lich by Morganthe, he is much more powerful than he was originally and he is completely invulnerable to attack and the only reason that the player is able to win the fight is the damage done to the battlefield while killing his minions causes the ground to break under him when he tries to finish the player off.
In Devil Survivor, there are two fights against the vampire Kudlak. If he's killed in the first one, he gains a level boost for the second. If you kill him in the second battle, a sympathetic character is Killed Off for Real. [[note]]You're supposed to let Mari kill him, as she's currently under the effect of his Kryptonite Factor and can make him stay dead this time. She's present in the first battle too, but the 'kryptonite' is not.
Pretty much the game of Prototype, Alex Mercer starts as a normal human but after you die you fall onto the virus you unleashed, gaining super strength, speed and pretty much invincible. Of course it turns out that he never did come back, this is The Virus wearing a Alex Mercer suit.
The game has an... Interesting take on this trope. The Exile is a former Jedi who has 'died' by losing all access to the Force and only regains it through a bond with Kreia, another former Jedi who has 'died' ideologically, twice. In Star Wars the Force pretty much is life/God, so to lose all access to it is a state most Force-sensitives would see as equivalent with death.
A more minor version happens to Kyle Katarn in the Dark Forces Saga. His version only goes as far as giving up the Force, but the Big Bad believes that his time away has made him stronger.
In the first game, Darth Malak expresses his disbelief that despite his betrayal leaving Revan half-dead and amnesiac one year earlier, he's not only rediscovered their training, but if returned to the light side, have somehow become even stronger than he ever was as a Sith.
The Undead of Dark Souls and Dark Souls II have this mixed with Came Back Wrong. On one hand, they gradually lose their memories and humanity until they turn into mindless Hollows. On the other hand, they're basically immortal, will not hollow until they have the will and purpose, apparently have no need to eat, drink and sleep and have the potential to aquire, through the use of souls, physical and/or mental prowess well beyond human limits.
In Avas Demon, the titular character forms a pact with Wrathia in order to do this.
In El Goonish Shive, the very first antagonist, the Goo, came back as the Omega Goo in the second arc complete with abilities related to the way it was defeated the first time.
In FreakAngels, all the titular characters have regenerative immortality and sustaining normally fatal injuries expands their already formidable Psychic Powers. Though they don't realize that until Luke gets back up after taking a bullet through the brain, previously they didn't realize that Arkady was more powerful than the rest of them following her overdose (she was rather spacey until she died again by drowning) and Mark didn't realize he had died. Then Connor shot himself to trigger the "upgrade" and gave it to the rest of them.
In The Order of the Stick, during his stay on the Mountain, Roy defeated an evil adventuring party with his dead grandpa, who later trained him in a sword move that can potentially one-shot a high-level spellcaster. He can't actually use it until he purchases the related feat, but when he returns to the living he says he has "this awesome idea for a cool sword move."
In Our Little Adventure, Jane came back to help her friends as a ghost. Her ghost powers included some nifty new attacks.
At one point in Schlock Mercenary, Kevyn Andreysan unknowingly has Project Laz'r'us blood nanites in him. They bring him back from the dead (or clinical death, at least) twice—on the second time, Kevyn authorizes an "aggressive reconstruction" that augments his body into a nigh-unstoppable killing machine.
Zebra Girl references Lord of the Rings by name when evil wizard Harold Duvase kills Jack and he returns to life moments later, much more powerful and calls it "pulling a Gandalf."
Vegeta: Unlike the runt [Gohan] and I, you don't get a power boost from it. Krillin: HAX!!! I CALL HAX!!!
Invader Zim had an episode featuring a plot by Zim to do in Dib with time traveling stuffed pigs. It was actually working at first, but just at the point where Dib was almost finished off, the timestream apparently had enough of that premise and started transforming Dib into a massive cyborg murder ninja.
Parodied on South Park in which Cartman throws himself off a roof in a poorly-planned attempt to fly and wakes up from a coma in the hospital, and the cops who have the Idiot Ball believe he has precognition. Kyle later does the same thing at the end of the episode so people will believe him about the serial killer's identity and Cartman's uselessness. Eerily, the lights flicker violently when Kyle gets frustrated.
In the movie, the nearly-dead Megatron is thrown into space after his and Prime's final battle. Then he encounters Unicron and is reformatted into Galvatron, gaining power great enough to kill a Transformer and blow up planets with one shot.
The season two opener for Beast Wars ends with Optimus Primal coming back from the dead, complete with a Mid-Season Upgrade. He sweeps in and singlehandedly drives off the Predacon army.
Octus was killed in episode 18 of Sym-Bionic Titan and remained that way through episode 19. Octus is revived in episode 20 by the leader of G3. While he himself hasn't been shown to have become stronger, the Titan gets an upgrade as a result, gaining powerful energy beams and is generally overall stronger. Lance even comments that it's stronger than before.
Colonel H. Stinkmeaner from The Boondocks was just an annoying, foul-mouthed old blind guy, who got his ass whooped, and died as a result. Come season 2, we find out he's been spending his time in Hell (like he'd ever get into Heaven...) turning himself into a kung fu asskicker. When he gets sent back to Earth via possessing a young guy, he mops the floor with the Freemans.
Optimus Prime of Transformers Prime spent the start of the third season critically injured and slowly dying until Smokescreen used the power of the Forge of Solus Prime to fix him. He comes back in a new, stronger body, and has gained the ability to fly.
This is how muscles and bones improve. When you strain them the cells die, but they get repaired to be stronger than before.
Overtraining is possible, where you break down cells faster than you grow them. Many bodybuilders and strength athletes deliberately overtrain when they need a large amount of development over a short time or when they expect to miss several exercise sessions.
This is why students of the Okinawa school of Karate routinely hit themselves with bricks. Hitting bones repeatedly causes microfractures, which then heal to form a stronger, denser lattice, resulting in stronger bones. (It also helps to desensitize the nerve endings.)
Many systems of martial arts do something similar: special mention to Cimande Pencak Silat where the whole system is about overpowering wrist or forearm strikes, and the practitioners would originally train against trees to get the conditioning. Though there is a lot of controversy about this topic—some say it works fine, some say isn't useful, some say it will give you problems in later life, and some say it doesn't work at all.
Evolution sometimes works this way. If something kills off most of a population (or species), but several individuals have a genetic trait that helps them, then the surviving population, while smaller, will mostly have the trait that makes them stronger. The population/species can then come back, more numerous and stronger than ever before.
Weird as it might sound, the entire human species is an example of this trope, not once but twice. They evolved 200,000 years ago. 150,000 years ago, we escaped extinction by a narrow margin. What happened afterwards? They expanded from their little nest to most of Africa. Then 74,000 years ago, history repeated itself and the human population dropped below 20,000 individuals. So what did people do next? They took over the frigging world.
Human enemies too: After a century of antibiotic use, bacteria are now more resistant than ever, with some resistant to virtually all known antibiotics; the same with antiviral drugs, pesticides, and herbicides.