There are tons of dark prophecies of doomsdays and master plans that take a long time to put into action. The Day of Reckoning is the day that all the prophecies speak of... the day that the Evil Overlord wins, goodness and mercy are overthrown, and a new Dark Age arises on Earth...
At least, until the hero shows up, defeats the villain, saves the day, and returns everything to normal (usually with one or two characters sacrificing themselves to do it). This trope is about the final confrontation between the heroes and the coming evil. The long-awaited and foretold apocalypse. When the heroes finally get out from under the Darkest Hour and put a stop to the villain's plan for world domination.
And nope, this trope has nothing to do with Universal Soldier: Day of Reckoning despite its title.
As an Ending Trope, expect massive unmarked spoilers ahead.
- Puella Magi Madoka Magica: The Walpurgisnacht. And Homura has faced it too many times to count. And losing, every single time.
- Digimon Adventure had the arrival of Myotismon be "The Hour of the Beast" or 6 seconds past 6:06 PM.
- At the end of Fullmetal Alchemist, this pretty much happens when Father takes the lives of everyone in Amestris, only to have Ed and everyone else foil his plans, and he was eventually defeated.
- Star Wars: The day Emperor Palpatine takes over the Republic, beginning with the murder of all the Jedi, at the end of Revenge of the Sith.
- The Battle of the Five Armies at the end of The Hobbit.
- The Battle of the Black Gate near the end of The Lord of the Rings, used to divert Sauron's attention so that Frodo can do his job.
- The Battle of Hogwarts, in Harry Potter and the Deathly Hallows.
- The poem Dream of the Rood briefly speaks glowingly of the Second Coming of Christ.
- The Dark Forest invasion in Warrior Cats, which is spoken of in several prophecies, and had been built up for most of the series.
- The battle in the Gaunt's Ghosts novel Sabbat Martyr is a Double Subversion. The stage is clearly set, we are told, for a dramatic, winner-take-all battle between Imperial Crusade forces and the hosts of Chaos at the fortress-world of Morlond, and the eponymous long-dead Imperial Saint has returned to spur the Imperials on to final victory. But instead of going to Morlond, Sabbat takes a detour to an obscure, worthless planet on the Crusade's second front called Herodor, and neither Gaunt nor anyone else can explain why. A massive Chaos army, led by Magister Enok Innokenti, attacks Herodor, and only then does Sabbat explain her plan: she deliberately lured Innokenti to Herodor, using herself as bait, to prevent him from attacking the Crusade's vulnerable flank, and the prophesied victory is an Imperial counter-attack that slays Innokenti and throws his army into rout.
- Dollhouse is notable in that its day of reckoning goes ahead despite (or possibly because of) the heroes intervention. Other than that, it plays this trope to a T.
- When Angel and his team finally attack the Circle of the Black Thorn and try to avert THE apocalypse.
- The new Doctor Who series had a Day of Reckoning at the end of every season. Of particular note are the episodes "The Sound of Drums"/"The Last of the Time Lords", "The Stolen Earth"/"Journey's End", and "The End of Time".
- Any of the Apocalypses from Buffy the Vampire Slayer.
- In Supernatural, we have the episodes "All Hell Breaks Loose" where the brothers finally take out Azazel, the demon that killed their parents... but are too late to stop him unleashing his demonic army into the world; No Rest For The Wicked where the brothers attempt to take out the demon's new leader, Lilith, before she can claim Dean's soul and drag him to hell... and they fail, with Dean suffering a particularly nasty death; Lucifer Rising, where the brothers attempt to stop Lilith from freeing Lucifer... and it ends badly, with Sam accidentally freeing Lucifer himself. Then there's the big one, Swan Song, where the brothers have to re-imprison Lucifer before he can fight Micheal, ending the world... they succeed! (Although Sam is sealed in Hell along with a very pissed-off Lucifer and Michael)...
- In Last Gladiator, the "Warlock" table is a magical battle on Judgment Day.
- Judgment Day, as described in the Book of Revelation. This makes this trope Older Than Feudalism.
- While the judgment before the great white throne is literally a day of reckoning (since "reckoning" in this sense is synonymous with "judgment"), the battle of Armageddon corresponds more closely to the trope meaning. The good guys have already won by the time Judgment Day comes around.
- Ragnarok, the prophesied final battle between the gods and Jotun in Norse Mythology.
- Call of Cthulhu campaigns
- Shadows of Yog-Sothoth - The rise of R'lyeh and release of Cthulhu. 'Nuff said.
- Masks of Nyarlathotep - The opening of a Gate that will allow Cthulhu Mythos entities to enter our world.
- The Fungi From Yuggoth. Dholes will be summoned worldwide, the Sphinx will be changed into an avatar of Nyarlathotep and terrorists will overthrow national governments, allowing the Mythos to take over the world.
- The Day of Lavos from Chrono Trigger.
- In Persona 5, the game's final dungeon, Qliphote World occurs on the Day of Reckoning.
- In Devil Survivor 2 the Septentriones are prophesied to descend on Earth, one at a time, for a week, while literally deleting the whole world, after which Polaris will grant the wish of the survivors in the method to reconstruct the world. Alternatively, you may either decide to defeat Polaris yourself or assist Alcor to ascend the heavenly throne.
- In God of War III Kratos at last brings fourth the destruction of Mount Olympus, and the gods upon it, as all was foretold.
- In Shadow the Hedgehog the first thing villain Black Doom says upon the arrival of himself and his forces is the promise of such a day:
Black Doom: Shadow... As you can see, the day of reckoning will soon be here. Find the seven Chaos Emeralds and bring them to me as promised.
- The Final Day from The Legend of Zelda: Majora's Mask.
- The Warcraft series had this with the invasion of the Burning Legion, though they've tried and failed three times already. Rumor has it, though, that they're at it again.
- In World of Warcraft, the climaxes of both Wrath of the Lich King and Cataclysm in the Icecrown Citadel and Dragon Soul raids respectively. It isn't so much that the villains are planning anything specific at that particular time, but with so many of the allied forces on the line, they'll be home free if the heroes fail.
- The Mass Effect series has one of these at the end of each game: in the first game, it is the defense of the Citadel and in the second, quite predictably, the Suicide Mission (see the opening quote). The entire third game is more or less an extended one of these, and the assault on Reaper-held Earth serves as an internal example within that.
- Trigon's invasion of Earth in Teen Titans.
- Sozin's Comet triggers this in Avatar: The Last Airbender
- Also the Day Of Black Sun, in a sense — it was supposed to be the allies' day to strike at the Fire Nation, but Azula discovered their plan beforehand and they were resoundingly crushed (except in Omashu).
- The Legend of Korra has Vaatu's return, which although occurring every ten thousand years, is treated as such.
- In Transformers: The Movie, the coming of Unicron.
- The two-parter Second Season finale of X-Men: Evolution is actually titled "Day of Reckoning." It involves the iconic X-Men figure of a Sentinel being let loose in the city and attacking all mutants and them having to defend themselves, leading to their existence being revealed on national television.
- The two-parter series finale "Ascension" probably also counts, as the titular heroes fight against and defeat their ultimate enemy Apocalypse, who had been prepping up for the last two seasons to turn all of humanity into mutants.
—Riley Finn, Buffy the Vampire Slayer