The new Big Bad Yhwach and the emperor of the Vandenreich from Bleach was once sealed by Shigekuni Yamamoto-Genryuusai, but he "regained his heartbeat again after 900 years, regained his intellect after 90 years and regained his power after 9 years". Now Yhwach is back and he's going to "regain the world in 9 days".
Captain America, who slept for Xnote Sliding timescale means that X=the amount of time between 1945 and about ten to fifteen years ago years until our greatest need...
In a Iron Man story featuring Doctor Doom and Time Travel, Stark and Doom find themselves in a future England (this was a sequel to a earlier storyline that had seen the same two characters go back to Arthurian times). Merlin is back, as is Arthur. Only due to genetic engineering and such Arthur was literally reborn to two Yuppie Britons and so is a spoiled young brat. So guess who has to take his place?
Camelot 3000 takes the King Arthur legend described above and runs with it. Arthur does indeed return in the hour of England's greatest need: an alien invasion in the year 3000.
In DC One Million and All-Star Superman, our Superman goes into the sun in order to rebuild its heart and leaves the superheroing to his many descendants who he blesses with extra-extraordinary powers. He returns after fifteen thousand years and brings New Krypton into our solar system.
In The Books of Magic, Tim Hunter and Doctor Occult encounter The King Under The Mountain. When they ask which king, they're told he's all of them. The bard under the mountain specifically name-checks Barbarossa and Arthur, among others.
The Elseworlds story Batman: Dark Knight of the Round Table re-imagined Batman as a knight in King Arthur's court. At the end of the story, the dying Batman is enchanted to sleep and awaken at the hour of Britain's greatest need. the final page shows the Batplane battling German fighters during the Blitz.
Suur Toll, being an animation of the myth of Toell The Great, has his decapitated head announce that he will one day return to protect Saaremaa, but without those troublesome kids mucking it up.
In the director's cut of Army of Darkness, Ash does this, complete with his car and boomstick, but wakes up 1000 years later than planned and misses humanity's demise.
In Little, Big, Frederick Barbarossa re-awakens. Subverted since he's a Knight Templar who, when elected President of the USA, plunges the country into a dark age of tyranny, economic depression and civil war.
H. G. Wells' The Sleeper Awakes, where a man previously in a coma for two centuries happens to awaken to find himself in a bleak, dystopian London of the future. Not only that, but upon first entering his trance-like coma two hundred years prior, his money had been placed into a Trust which had managed his money for him in his name; this money accumulated into a vast quantity over the centuries, due to the compound interest compiling steadily for so long, and the stewards of that Trust eventually put all this wealth to establishing a global political and economic conglomeration. The effects of all this boil down to the Sleeper now finding himself the richest man in the world, as well as effective leader of the world seeing that this political and economic entity had been created in his name. He proceeds to assume the role of the "Hero" in an attempt to restore London from this grim present and free the oppressed populace.
The Army of the Dead in The Lord of the Rings - traitors cursed to guard the tunnels under the White Mountains (the Paths of the Dead), until they fulfilled their promise to protect Gondor.
In The Silmarillion, the evil last king of Númenor, Ar-Pharazôn, set sail with his armies to conquer the holy lands of Valinor from the Valar (god-like angelic demiurges). The Valar prevented the heresy, and possible slaughter of resident elves, by burying him and his armies under a mountain. Supposedly he will only be freed to fight in the final battle with the Dark Lord.
The graveyard in The Graveyard Book was built over the tomb of a king like this; he never actually wakes up, but the spirit guarding him plays a crucial role in the plot.
In Blood and Iron by Elizabeth Bear, King Arthur is actually awakened (and ends up dying in a Heroic Sacrifice).
It's also evoked in The Merlin Conspiracy, although in that case Arthur 'The Count of Britain' is just one of several forces invoked and doesn't stick around after the upset any more than The Wild Hunt does, apart from scolding the current king.
In Poul Anderson's Orion Shall Rise, the line "Orion shall rise" is used by many citizens of a subjugated land. This trope is invoked to explain their superstition. In reality, they're talking about the restarting of a secret program codenamed "Orion". This was to ensure that any leaks would be attributed to the superstition, helping cover up the program. Given what the Orion Project is, the secrecy is understandable.
In Terry Pratchett's Lords and Ladies, we catch a glimpse of an old king and his warriors in a cavern under the Long Man. Some old wizard put them in a magical sleep from which they're supposed to wake up in time for some final battle when a wolf eats the sun. You can wake them up prematurely by banging a nearby bell, though they'll be pretty cranky about it. Haven't had a wink of sleep for 200 years.
A somewhat more sinister example would be the Elf king (different from the guy mentioned above), who waits beneath the Long Man for a time when "The iron in the head has rusted", which is to say, when there are no humans left capable of opposing him, at which time he'll take over the world.
"Big Fido", a version of this for dogs found in Men at Arms. The members of the Dog Guild assure themselves that when Big Fido comes back down from the mountain, he'll come and he'll bring all the knowledge of the wolves with him and then, then the revolution will start. Nevermind that Gaspode saw Big Fido reincarnated as a muff and hat for Foul Ole Ron shortly after watching Fido fall off a six-story building.
In the Farseer Trilogy, legends state that King Wisdom awakened the Elderlings to defend the Six Dutchies. He vanished afterwards, but will return again to save the land. After King Verity does the same to end the Red Ship war, the same legends form around him.
That Hideous Strength: the fact that Merlin is resting beneath Bragdon Wood is a major plot point, and both factions wish to recruit the reawakened wizard to their side.
The Silver Chair: we meet the Giant Time, who is lying asleep underground. This one's a subversion, though, as when he wakes up in The Last Battle it's a sign of the end of the world. "When he slept he was called Time. Now he is awake he will have another name."
Prince Caspian: Peter, Susan, Edmund, and Lucy are all suddenly yanked into Narnia at the beginning of the book and discover over the course of the story that they are the King In The Mountain that was awakened by someone else.
Lewis also uses the 'beard growing around table' imagery of this trope for the Good Telmarine Lords who made it as far as the Island of the Star.
In The Weirdstone of Brisingamen, the wizard, Cadellin, is the guardian of the Cave of The Sleepers; wherein Arthur and his knights sleep, awaiting the Final Battle or some time of great need.
A legend in Stephen Hunt's The Court of Air. At the climax, someone exploits this belief with smoke figures.
A variant of this occurs in the Warrior trilogy set in the BattleTech universe with Morgan Kell, the famous founder of the Kell Hounds mercenary unit. As the story opens, it's been years since he's disbanded half his unit and retreated to a monastery after a fight against one Yorinaga Kurita (who likewise hasn't been seen in action since) for reasons unknown to the others, and it takes his old enemy's return onto the battlefield and the death of his brother, though he didn't plan for that to happen to bring him out of his retreat and have him take the reins again.
A more classic example from the same setting: the legend that General Alexander Kerensky and his troops were still out there, somewhere beyond the Periphery, and would one day return to save the Inner Sphere in its hour of need. Well — their descendants did eventually return, all right...
Played for laughs in Tom Holt's Who's Afraid of Beowulf? in which Viking King Hrolf of Caithness (god forsaken country—but it is my kingdom) and his band of heroes are disinterred in time to put an end to the attempt of their ancient enemy, the Sorceror King, to take over the modern world via magic—or as we call it, technology.
In Her Majesty's Wizard, first book of A Wizard in Rhyme, Matt is taken to a cave in the mountains where the legendary emperor Hardishane and his knights are waiting for the time when they will need to save the world from evil. They don't come back just yet; Matt just needs to be there for a special knighting ceremony. But there is an example of Waking The Sleeping Giant.
Hardishane is specifically mentioned to be Charlemagne's counterpart in the Merovence universe, so it's sort of inevitable that something that was a myth about Charlemagne in our world literally applies to his counterpart in a world where magic exists.
If you happen to be a Fish Person or some other kind of gibbering horror, Dread Cthulhu, dreaming in his sunken city of R'lyeh, waiting for the day the stars are right again, probably qualifies.
This story from Strange Horizons has King Arthur and his men show up in WWII as a squadron of fighter pilots.
In Jack Campbell's The Lost Fleet, the story is that Black Jack Geary will come to save them all. This makes it rather awkward for Geary when he actually shows up. Especially after he does save them all.
In Maurauders of Gor, Torvald, legendary founder of Torvaldsland, is said to sleep in his mountain, awaiting someone to wake him to defend Torvaldsland. Ivar Forkbeard goes there and discovers that it's only true in a metaphorical sense.
In Patricia A. McKillip's Alphabet of Thorn, the Dreaming King will wake to save the kingdom. In the book, he wakes only enough to give the current queen a cryptic warning.
The Dark Is Rising series book The Grey King. To fulfill a prophecy of the Light, Will Stanton plays a golden harp and summons the Six Sleepers from their resting place beneath a mountain so they can ride against the Dark.
King Arthur comes 'back' to fight the Dark later in the series, but he's nowhere as specific, physical, or dull as under a mountain.
In Michael Flynn's The January Dancer, Hugh sneaks off planet with the promise to return again. Later, the Fudir speaks of the legends of Stonewall and how they correspond to many King in the Mountain legends.
In the Captain Future novel Planets in Peril by Edmond Hamilton, the Captain is convinced to go into a parallel universe and impersonate an ancient hero who promised to come back when needed. In the end, it is revealed he didn't go into a parallel universe, but his own twenty billion years in the future, and he was the hero he impersonated
Cthulhu counts. Awaits dead and dreaming? Check. Resides in his strange and otherworldy city? Check. Will return one day in connection to the apocalypse? Check, check, and triple check.
Live Action TV
In Star Trek: The Next Generation, there's a legend like this surrounding the Klingon imperial founder Kahless. In an attempted power grab, a group of monks make a clone of him and claim that he's returned.
Subverted in the Doctor Who serial Battlefield: it's implied that the King Arthur of a parallel universe is doing this on the main Whoniverse Earth, but it turns out he was dead all along and the story that he would return was just propaganda.
In Highlander: The Series Duncan became this in his hometown after his first fight as an immortal where he "killed" the immortal Kanwulf who was attacking his clan. These events started a legend that Duncan Macleod would return whenever his hometown, Glenfinnan was in trouble. When Kanwulf returned some time later, Duncan kills him off for real.
"In The Hall Of The Mountain King" by Edvard Grieg, used in many films and videogames (Elite 2 being a famous example).
Myth & Legend
King Arthur in Avalon, and Merlin in the oak. Said to come back when England is in its hour of most need. Still hasn't shown up, despite the threats of the Spanish Armada, Napoleon and the Nazi Germany. Since then Francis Drake, The Duke of Wellington and Winston Churchill have all been referred to as reincarnations of Arthur, and Admiral Nelson is also a candidate, though he'd have to mud-wrestle The Duke of Wellington to figure out which of them it was.
Bendigeidfran (or Brân the Blessed, not to be confused with BRIAN BLESSED), giant and King of Britain. His severed, still living head was buried under the White Hill in London, facing France, to protect the Island of the Mighty from invasion. A jealous King Arthur dug it up, claiming his strength alone was enough to safeguard Britain.
Charlemagne in the Untersberg near Salzburg. His beard is growing around a table, wrapping twice around it so far. When it has reached around three times... you know the deal.
Ogier the Dane (Holger Danske), asleep under Kronborg castle, near Helsingør, Denmark. note Does this mean he was directly underneath the goings on of Hamlet? His role is summed up thus: "Når Danmark stander i våde, så vil Holger Danske vågne op til dåd" ("When Denmark is in peril, Ogier the Dane will rise to the occasion"). The statue of him in Kronborg is, in fact, a copy used for the casting (originally of plaster; later replaced by a concrete version) of the actual bronze statue. The real statue is located at a nearby hotel/casino named Marienlyst, and far from as well known as the copy. Local folklore of the area is that Denmark is only in serious trouble if bothstatues wake up. As Ogier the Dane was originally from a French story, where he served a French king, some versions of the stories had him be spirited away to Avalon by Morgan Le Fay only to return 200 years later to save France.
Friderich Barbarossa, asleep in the Kyffhäuser mountains in Thuringia, Germany.
Bernardo Carpio, rightful king of the Philippines, allegedly chained up hand and foot within the mountains of Montalban, and also caught between two boulders trying to crush him. He is often said to be the reason for the earthquakes in the area. The legend states that he was a Herculean man who fought against the Spanish occupiers, and so they enlisted a local engkantado (sorcerer) to trap him in the mountains. When his final chain breaks, so the tale goes, "the enslavement and oppression of the Filipino race will be replaced with freedom and happiness".
The Philippines' national hero Jose Rizal is said to be sleeping within Mount Banahaw, where he was supposedly (according to the Rizalistas) spirited away after his execution in 1896. A local cult called the Rizalistas revere him as God made manifest, and believe that he will return from the mountain to make the nation great again, riding upon a white horse and his appearance heralded by seven suns rising over the Philippines.
Sir Francis Drake is said to be sleeping "in his hammock" on the sea floor. The legend says he'll return in England's time of need, when his drum at Buckland Abbey sounds. While he may never have returned in body, legend has it that the drum has sounded by itself several times over the centuries.
Väinämöinen. He's apparently not sealed in any single location, but wanders between the stars ("higher earths, lower skies") until he's needed again. Parodied in a Finnish comic book when Väinämöinen visits Avalon. He meets King Arthur, mentioned above, and they briefly discuss this trope. Arthur reveals that he did in fact return once, during World War II, before adding that it took him two years to escape that asylum.
Gearoidh Iarla (Earl Gerald) and his warriors are asleep in their seats around a long table under the Mullaghmast. Once every seven years, the Earl awakens and rides his horse around the Curragh, in County Kildare. When the horse's shoes are worn "as thin as a cat's ear", he and his warband will arise for good and drive the English out of Ireland.
The twelfth Imam of Shia Islam lived most of his life in a cave, was (allegedly) hidden away by God, and is expected to return someday to establish justice.
Constantine XI, the last Roman Emperor, who in a variation was turned into a statue instead of sleeping, waits to be awakened and help take back Constantinople.
Sebastian I of Portugal was widely rumored to have survived the battle in which he was killed. The myth that he would return to save Portugal in its darkest hour persisted and even spread to Brazil.
King Wenceslaus (yes, the one from the Christmas carol, though he was actually just a duke), in Czech folklore, who sleeps inside the mountain Blaník, in the Czech Republic. With all his knights, too. They are supposed to show up and whip away adversaries when the country is at her worst. They haven't shown up even though the country had been oppressed terribly and there are several extremely poignant moments in the history. The motif of them coming to help recurs in literary works, especially in poetry.
King Matja, who was presumably based on King Matthias Corvinus. In Slovenia, he is a legendary hero who sleeps with his army in the mountain called Peca in the Karawanks. When his beard grows nine times around the table, he will awake from his sleep and bring a golden era to his people.
Not a King per se, but Fenibeso was the first sole ruler of Okrika (in today's Nigeria). Local religion states that he did not die, but instead went into the jungle, became a spirit, and is now the kingdom's patron saint and god of war. The belief is that he will return physically to lead Okrika's armies if she is ever on the verge of being conquered (apparently getting colonised by the British didn't count because the reigning monarch signed a treaty willingly before a shot was fired).
Norse Mythology has the villain version of this for Loki. His return will bring the end of the world. However, the dead god Balder is also meant to return at the same time, and help to rebuild from the ashes.
According to Japanese folklore, the 9th-century Buddhist monk Kukai (Kobo Daishi) is not dead, but "meditating" underneath Mount Koya, awaiting the coming of Maitreya. His followers bring him food and a change of clothes daily, and a massive cemetery has sprung up around his resting-place.
The giant-king of Estonian myth Toell the Great was supposed to be this for Saaremaa, but a bunch of troublesome kids lied and said there was a war, so he swore to never return.
According to the epic Kalevipoeg, Estonia's mythic king Kalevipoeg will one day return.
According to a Polish legend, there is a whole army of knights (whether a king is included varies depending on the version of this legend), sleeping in a cave under the Tatra mountains until the time of need comes. Every now and then, someone stumbles into a cave. The king asks them "Is it time?" and they have to explain to him the country's situation. If he finds it desperate enough, the army will ride out to fight the invaders. If not, he concludes that it is not, in fact, time, and he resumes his wait.
According to a Serbian legend, Marko Kraljević, a legendary warrior, went to sleep long ago and will rise again one day to a better age, when he will once again kick ass and take names. The reason why? He saw a gun and realized that "the weakest sissy could kill the bravest, strongest warrior with it" and decided that he was just too awesome to go down like that.
A modern legend related to this trope involves Subhas Chandra Bose, the Indian independence leader who escaped British India for Germany then took two submarines (one German, the other Japanese—they rendezvoued off the coast of Madagascar so that Bose could change ships) to Japanese controlled Southeast Asia. Supposedly, he died in a plane crash in Taipei, Taiwan, on August 18, 1945, but he is said to have survived the crash and either became a Hindu mystic or somehow took refuge in Soviet Union. Some (including his relatives) have claimed that he would return to an independent India (although no one has quite claimed that he would return to lead India at time of its need).
In Warhammer 40,000, the God-Emperor of Mankind has been confined to the arcane life-support systems of the Golden Throne for ten thousand years, and there's a number of theories and heresies about the circumstances of His possible revival. Some believe He will rise again one day to begin a new Great Crusade, while others hold that if everyone would just let His ruined husk die, the Emperor could reincarnate into a healthy new form. The Inquisitor rulebook mentions a theory that the Emperor could have been recalled to His body as early as a year after being placed on the Throne, but those ruling in His place prevented it to preserve their own power and the stability of the Imperium. In any case, He's pretty much stuck there while the setting goes (further) to hell.
This is a common legend regarding the fate of certain Space Marine legions' Primarchs. Roboute Guilliman of the Ultramarines was put in a stasis tomb after being terminally poisoned, but nevertheless the chapter maintains that he's slowly healing and will fully recover someday and because Jaghatai Khan of the White Scars and Leman Russ of the Space Wolves both disappeared while pursuing enemies instead of being confirmed dead, they're expected to return for an apocalyptic final battle. There's even a similar myth about Ferrus Manus of the Iron Hands, who was not only confirmed killed but decapitated during the Horus Heresy.
Though there are hints that Leman Russ is still alive. The Space Wolves have a legendary 'lost company' who are believed to have accompanied Russ, and one of the Black Crusades threw up mention of potential sightings of gigantic, ancient Space Marines fighting in the Eye of Terror itself against Chaos and their leader was said to be a giant, even among the Marines.
If any primarch could survive 10,000 years fighting in the heart of Chaos, it'd the one who could punch the Emperor across the room with his bare fist while the Emperor, the greatest warrior in all of human history, possibly the universe, able to kill thousands with little more than a thought, while he was wearing the greatest suit of armour ever forged by man.
The legend about Leman Russ causes problems in Grey Hunters. The Spear of Russ is the Space Wolves' most cherished relic, prophesied to have been left behind by the Primarch for his return. So when Ragnar destroyed the Spear by using it against a resurrecting enemy, even Ragnar, convinced that he did the right thing, is disturbed by the idea of his Primarch not having the Spear for the destined battle.
Lion El'Jonson, primarch of the Dark Angels, is a literal case since he is sleeping in a hidden chamber of the Dark Angels asteroid headquarters to wait for the final battle. In an interesting inversion, the Dark Angels aren't aware of this and actually believe that his body disappeared during the final battle with Luther. The only people who know the truth are Luther (who is mad), the Emperor (who can't talk) and the Watchers in the Dark (a race of mysterious creatures that either can't or won't talk).
In Henry Zhou's novel Emperor's Mercy, the ancient xenos artifact the Old Kings is supposed to produce the Star Kings at some point.
The dwarfs of Warhammer believe that their ancestor gods Grugni and Vallaya disappeared into the heart of the mountains, to return when needed. Dragons also have a habit of hibernating for centuries at a time (they're not adapting to the planet's changing climate well), and can only be roused by powerful mages.
Breath of Fire IV subverted it to the point of deconstruction with Fou-lu. He was meant to be this trope if things goes as planned, if the Fou Empire (which he founded) wasn't corrupted to the point of wanting him dead. Instead, Fou-lu's entire storyline in the game can be best described as "What Happens When A Country's Government Sees The Return Of Its King in the Mountain As An Unwanted Revival". It goes poorly for all involved.
Commander Shepard inbetween Mass Effect 1 and Mass Effect 2, having spent over 2 years being brought back to life by the Lazarus Project.
Planescape: Torment: The Silent King of the Dead Nations sits frozen on the throne, a little more dead than The Undead citizens. Except during the game, he's actually completely dead, which Hargrimm is covering up. The Nameless One may choose to replace him, or give the position to a previously treacherous diplomat.
The Big Bad from The Elder Scrolls III: Morrowind, Dagoth Ur, is a villainous version. He was thought to have been vanquished, but having already attained godhood, his defeat was only temporary and he regained his power over several millennia leading up to the events of the game.
In the backstory of Drowtales Queen Sharess made a promise to the Dark Elves that she would return and lead them back to the Surface from the Underworld once it was safe again after demonic wars had made it uninhabitable before she departed into the Netherworld to seal the gates. After 1000 years there are no signs of her returning and the Dark Elves and those who believe in her are largely extinct outside of the Kyorl'solenurn clan. However, Sha'sana actually has been keeping Sharess' body the entire time and has her own plans to bring her back to unite the clans.
In Beast Wars, the original Optimus Prime (in stasis lock) sits in his command chair in the Autobot ark, which crash-landed and buried itself under a dormant volcano, awaiting a revival millions of years in the future.
In The Boondocks, Dr. Martin Luther King Jr. is revealed to be this, awakening from a coma 40-odd years after being shot. In a pretty dark subversion of the trope, he turns out to be pretty disappointed with the direction that African-American culture has gone in his absence.
As with the legend, this happens to Gargoyles' version of King Arthur. And when King Arthur came out, the Magus replaced him after using so much magic without a channel like the grimoire.
In X-Men: Evolution's take on Captain America's origin, his role as a King in the Mountain is made even more explicit. Instead of being accidentally frozen in an iceberg and presumed dead for years, he's intentionally placed in cryogenic sleep when it turns out that the super-soldier serum is slowly killing him. The implication is that he will be revived to fight again when S.H.I.E.L.D. scientists find a way to cure him.
On The Venture Bros., it's lampshaded how Nazi's are simply obsessed with cloning or resurrecting Hitler in one way or another. As mentioned in the Real Life section below, this may count as an evil version of the trope.
There were real some historical figures who did remain in hiding for a long time and presumed dead, only to resurface when the situations seemed right. An example of this is the Cossack Ataman Nikolay Kulakov, an anti-Bolshevik leader from the Russian Civil War who lost both legs and presumed dead, only to have remained in hiding for two decades, until he volunteered his services to the invading Nazis. The story did not end well.
Legend holds that the last emperor of Byzantium (Eastern Rome) did not die when Constantinople was taken by the Ottomans, but rather, turned into a statue, and will return when the city is in need of him again.
There is another, lesser known and less popular urban legend in which John F. Kennedy actually survived being shot but has been in a coma somewhere for all those years. Camelot indeed.
The earliest American case of this was probably Andrew Jackson. After founding the Democratic party and becoming by far the most powerful President up to that point, he retired to a life of relative obscurity. Naturally this lead people to believe he would one day take the political reins of America to solve some situation, and his death did little to dissuade them. In the months leading up to the civil war many believed Jackson would rise again and resolve the whole situation.
Similar rumors sprung up about Michael Jackson faking his death. There is one theory claiming that Elvis was abducted as a representative of Earth to some aliens (because he was "the king") and when he died they came back and got Michael Jackson to replace him.
Some people believe Tupac Shakur will return in the (insert number here)-th anniversary of his death.
There are similar myths regarding the return of Andy Kaufman.
Belief may have waned over the years, but for some time, assassinated populist Louisiana governor Huey Long was expected to return to lead the state to prosperity.
Played with with Harold Godwinson. According to one story he was buried by William's permission by the coast his army had camped at at the start of the campaign with an epitaph invoking his eternal guardianship over the English coast. This tale doesn't seem to have found a large place in British folklore though.
Played straight by Bohemian folklore; King Wenceslas is said to sleep under the mountain Blanik, with a massive army of knights. Legend holds that he will return to defend the country in its hour of need.
Welsh rebel Owain Glyndŵr supposedly will return from hiding when Wales needs him the most.