The standard phrase when the villain finds that he has been defeated by the heroes and there is no point in staying around with the immediate Evil Plan foiled.
In effect, the villain shows a Determinator streak and threatens the heroes that he will return to fight them again, and he is sure that he will defeat them next time.
If the villain is in a more poetic mood, they might say their parting words with, "He who fights and runs away/Lives to fight another day." Other variants include: "I'm not finished with you," "This isn't over," "You think you've won!" and "You haven't seen the last of me!"
Often times, the heroes will respond confidently, "We'll be waiting!" (He obviously has never heard of Villain Decay.)
In some rare cases, the villain with Medium Awareness might breach the Fourth Wall by saying: "I'll get you next week", if it's a weekly series. The hero is usually not too concerned.
Also the standard exit of the Enigmatic Minion during his 'I am not here to fight you today' appearances.
See also Villain Exit Stage Left, We Meet Again. Compare to I Shall Return, a typically more heroic trope.
In episode #141, as the Arrancar are drawn back to Hueco Mundo they tell their Soul Reaper opponents that they will meet (and fight) again. Except Luppi; oh poor Luppi...
In episode 319, after the Big Bad Kageroza Inaba is defeated and is about to retreat to the Soul Society he tells Ichigo that they will meet again.
Mahou Sensei Negima! has a rare heroic example: At the end of Chapter 59, Yue swears revenge on the dragon under the Library, mainly because he drooled on her.
Naruto: In a rather bizarre inversion, Hanzo tells Jiraiya, Tsunade and Orochimaru to "come back to fight me again", after he won the battle. They're among the few people Hanzo considered to be Worthy Opponents, along with the Samurai Mifune. Except that he doesn't live to fight the rematch. He does, however, get a rematch with Mifune... after being raised as a zombie.
A heroic example occurs twice in One Piece. The first time, Jimbei tells Luffy that they will meet again on Fishman Island. The second time is when he said he will be back to join the crew.
Happens to the manager of the illegal casino from the second part of Kaiji. After the casino loses lots of money as the result of Kaiji's actions, he is sentenced to spend the rest of his life in a underground labour camp; as he is being taken away Kaiji yells him to make his way out and come back to challenge him; his response: "Isn't it obvious that i will!? You better prepare yourself until then!".
Pokémon episode "Pikachu Re-volts": "This isn't over."
Digimon: Myotismon does this a lot. Ken decides to back off after revealing himself.
In Soul Eater, Anti-Hero and Ninja Sid goes up against Anti-VillainSamurai Mifune. When the heroes are ordered to retreat, Sid tells Mifune that he has no more reason to fight him, but warns him that if he lets him go Sid might come for him and Back Stab him some day. Mifune's only reply is "Sounds fun. Go."
A rather terrifying example from Berserk (which is scary enough by itself): every single time Guts drives the Beast out of his mind, the Beast makes it clear that it will return sooner rather than later.
Luann. Brad's friend T.J. managed to get Ann Eiffel out of her job of manager at Weenie World. When he told Brad about it in the strip for January 12th, 2013, he said that her parting words were "You haven't seen the last of me!"
The heralds tend to make comments like this when defeated.
Films — Animated
In The Incredibles, after his kidnapping of Jack-Jack is foiled, Syndrome stops to shout, "This isn't the end of it! I will get your son eventually! I'll get your son!" This is a mistake: it prompts Mr. Incredible to end it right there by throwinghiscar at Syndrome's plane.
At the end of An American Tail, after the Big Bad has been knocked onto a ship for Hong Kong, he vows, "Someday, Gussie Mausheimer! Someday! And you too, Mousekewitz! all of you! I hate mice!"
Films — Live-Action
Robin Hood: Men in Tights: Parodied/subverted: the leader of a band of henchmen calls out "You've not seen the last of us!" as he prepares to retreat. Robin fires multiple arrows with a single shot and pins him to a tree. Pinned Mook: "You've seen the last of us!"
In Gladiator, Maximus promises Commodus that he will have his vengeance, "in this life or the next."
The Green Goblin actually yells out "We'll meet again, Spider-Man!" after their first battle in the Spider-Man movie. Considering how straight-faced the movie is otherwise, hearing such a corny line thrown in was really weird.
Tank Girl: After Tank Girl drops a bucket of water on Kesslee, he tells her "It's not over."
Legend: After Jack blows Darkness away with sunlight, Darkness says "You think you've won."
The Great Race. "He who fights and runs away/Lives to fight another day.": Baron Von Stuppe to The Great Leslie after being defeated in a sword fight. Made even better by Von Stuppe completely bungling his escape plan a few seconds later.
Odd heroic usage in Terminator 3: Rise of the Machines. Just before the good terminator blows up the Terminatrix with his own power cell, he tells John Connor, "We will meet again." The twist, in this case, is that the Terminator sent back in time is the one that succeeds in killing John in the future.
Boris: What happens after we die? Is there a Hell? Is there a God? Do we live again? All right, lemme ask one key question. Are there girls? Death: You're an interesting young man. We'll meet again. Boris: ... don't bother. Death: It's no bother. And, of course, they do meet again in the end, dancing away as the credits roll.
In A Christmas Story, when Ralphie (in his imagination) shoots the bad guys, and Black Bart is escaping, Bart says, "I'll be back." Ralphie replies, "I'll be waitin'"
In The Mummy 1999, Imhotep corners the heroes, and is taking Evie away to be sacrificed. Before the Big Bad can leave, Rick O'Connell looks right into the Big Bad's eyes and says, "I'll be seeing you again." with all the menace and hatred and promise the man can muster. Imhotep, who doesn't understand a word (the man's an ancient Egyptian who doesn't speak English) get's the message anyway, but he's so assured of his near invincibility that he only reacts with mild amusement. Turns out, underestimating O'Connell was a huge mistake.
Septimus Heap: When Alther drove DomDaniel out of the Wizard Tower in the backstory DomDaniel said that he would return with the seventh son of a seventh son.
Tale of the Troika: At the end of the second edition of the novel, Vunyukov makes classical "We will meet again!" speech, delivered in his usual bureaucratic manner.
Live Action TV
Charmed has Zankou who initially appears to be a minor/one-shot character and then ends up becoming a major threat later on in the season.
The next of Niska's Mooks they bring up is a little more compliant, to say the least.
This is subverted repeatedly on Firefly, with Mal summarily dispatching characters that spend the episode seemingly being built up to be potentially recurring villains. However, this was only because the show only lasted a single season, and Word of God is that some of those characters were intended to survive and return for revenge on the crew.
One of them does in fact re-appear as a villain in a Firefly comic, only to be killed again, this time for good. (Mal shoots him twice, just to be certain.)
In "Androids of Tara", the defeated villain, Count Grendell, leaps off the battlements while warning "Next time, I shall not be so lenient!"
The Meddling Monk's last words on the show: "I'll get you for this, Doctor! I'll get you one day!" Over forty years later, he doesn't seem to have got round to it yet (except in the Doctor Who Expanded Universe). The Time War probably put a crimp in his plans.
Supreme Dalek: You will never defeat us, Doctor! We will return!
New Paradigm Daleks: WE WILL RETURN!
The Master never stays away for too long particularly in the classic series. In fact he's the villain in every third or fourth Fifth Doctor story.
In The Sarah Jane Adventures Miss Wormwood promises this somewhat randomly in the pilot episode. At the end of the second season she reappears, having upped her ambition from "ensure the Bane breed on Earth" to "conquer the galaxy" and needs to sucker Sarah Jane into helping her.
In Thunderbirds, the Hood is frequently heard to utter a variation on "International Rescue haven't seen the last of me!" after his latest plan ends in an explosion.
In the season finale of Blackadder II, "Chains", Prince Ludwig the Indestructible is stabbed and exposed by Blackadder, but doesn't leave before talking with Queenie, revealing that he was "the tall and attractive German schtablelad" that held her favourite pony in her youth and, finally, swearing revenge when called "Short Greasy Spot-Spot":
Ludwig: [Utterly dismayed] No! [Getting up and walking to the door in a rage] No! No! No! ... You vill all of you, regret the day that y'ever mocked my complexion! I shall return and wreak my rewengue! [Evil Laugh]
Blackadder: No you won't, you'll die and be buried! [throws knife]
Blackadder: Strange man.
He is presumed dead, but turns out to really be indestructible and still manages to slay the entire cast in the end. So this doesn't always fail.
Subverted in an episode of Buffy the Vampire Slayer: after an inconclusive skirmish with Buffy, a pair of dimwitted cowboy-vampires threaten her with "This isn't over!" Later in the episode, one of them gets sucked down by a giant underground tentacle-monster, which Buffy then cold-bloodedly hacks to death with a pickaxe. Witnessing all this, the survivor says: "All right. It's over!" and flees.
Although, in point of fact, they do meet again after that. That time Buffy kills his new vampiric wife, and he runs off again, without any threats. Maybe he'll be back in the comics.
He is, in "Chaos Bleeds". This time he yells "We're not done, traitor!" at Spike as he's pulled back into an alternate dimension.
The trope is lampshaded in "Band Candy" when Trick shouts, "You and me, girl! There's hard times ahead!" before fleeing. Buffy gripes, "They never just leave. Always gotta say somethin'."
Glory delivers a 'This isn't over' to Buffy and Willow in "Tough Love". Somewhat unusually, she actually follows through at the end of the same episode.
Turns out to be Tempting Fate in the Angel episode "The Girl in Question". Angelus discovers his Arch-Nemesis The Immortal has slept with his girlfriend and shouts, "This isn't over yet, Immortal! This'll never be over!" And indeed he and Spike keep getting stymied and cuckolded by the Immortal right up to the present day.
Jim Keats sings "We'll Meet Again" (the sentimental British song from World War II) to Gene at the end of the final episode of Ashes to Ashes.
In Sherlock Holmes, when Holmes has put handcuffs on Moriarty, whom the butler is about to drag him off to the police, Moriarty tauntingly suggests to Holmes that this might not be the end. "Are you quite sure the police will be able to hold me?" he says. When Holmes replies, smiling slightly, that he is sure of nothing, Moriarty slyly tells him that he has heard Holmes is planning to take "a little trip on the Continent" with Watson and will meet him there.
In the Universal Studios Singapore version of Revenge of the Mummy, when Imhotep is trapped once again in a sarcophagus, he assures the riders that he will return once again, reminding them that, "Death is only the beginning!".
Mardek RPG: Baron Von Doomkill swore he would return after being killed. An undefined but very long time later he is pulled from the antilife to animate Legion, along with Deena, Bostolm and Miserable Old Martin. Much to his frustration, the magic that summoned him binds him to the control of Mereador, so he doesn't get to even try to reconquer the world, despite being in Mardek's party while he gathers three of the six crystals.
Super Mario 64: Played with very interestingly. In the game's earlier fights with Bowser, you see Bowser saying Mario will pay for this... later. In these cases, it seems so distant; however, by the time of each next fight with Bowser, it tends to feel like it happened a lot quicker than it seemed like it would at the end of each previous fight.
Sonic Adventure: Dr. Robotnik sort-of does this, albeit in the middle of the three stories he does it in.
Ganondorf: "Curse you, Zelda! Curse you, sages! Curse you, [name]! Someday, when this seal is broken — that is when I will exterminate your descendants! As long as the Triforce of Power is in my hand..."
Ganon: "The chains! No! You haven't seen the last of me!"
A heroic version in Twilight Princess, where the Hero's Shade ends every meeting (except the last) with him this way.
In Age of Mythology, Kemos vows that "your head will hang on my mast, Atlantean!" just before he flees the first time you defeat him.
He says almost the exact same thing, in almost the exact same situation the second time, after which he tries escaping the same way he did the first time and ends up with his head smashed out on the rocks after you defeat him the second time.
In World of Warcraft the Lich King gets a version of this at the Wrathgate encounter. Faced with the combined forces of the Horde and Alliance, Arthas himself walks out, kills Saurfang the Younger, sucks out his soul and looks ready to take on the entire army facing him... when Grand Apothecary Putress unleashes the new Forsaken Plague, slaughtering undead and mortals alike. Weakened but far from defeated, Arthas falls to his knees and retreats back inside with a gasping "This... Isn't... Over!"
In the Dynasty Warriors series, defeating an opponent with a unique character model (or in some games, any with a name) will usually result in that character saying one of these before retreating.
After the second fight with her and her gang in Eternal Sonata, Dolce warns you that you haven't seen the last her and her gang. Indeed, there's another optional battle with them just a bit later in the game and yet another one if you're playing the game on Encore Mode.
Played with in the Final Fantasy VII prequel Crisis Core, when pre-insanity Sephiroth kindly reassures an anxious Zack that they will meet again soon. They do meet again... and head off to Nibelheim.
In Darksiders and its sequel, the demon Samael says this to War and Death after their business is concluded in each game.
In the "Captain Scarlett" DLC of Borderlands 2, Captain Scarlett says this after you kill Roscoe and she teleports away.
One of the more chilling parts of the Command & Conquer: Red Alert 2 base game Soviet ending. Yuri is still very much alive after you thought him dead, and makes his intent to return very clear. All he has do is find a suitable body to reupload his mind into.
Trigak: You may have won this round, Order of the Stick but we swear our revenge on you! When you least expect— [Belkar kills him] Haley: Belkar! [...] He was obviously supposed to be a recurring villain! I mean, he had a name! And he was in the middle of swearing to get us and our little dog, too. It was pretty obvious he was going to come back.
Also subverted in Super Friends in the episodes "Rub Three Times for Disaster" when the arrogant villain taunts the superheroes with the "He who fights..." couplet before making his escape. However, Wonder Woman immediately ropes the villain and hauls him back while saying, "He who fights the Super Friends is always captured in the end." (Except the Legion of Doom, that is.)
Vlad from Danny Phantom parodies this in his first appearance, which ends with him letting Danny go because he's threatening to reveal his identity, by acting over dramatic about it.
A heroic version is used when Wulf makes a Heroic Sacrifice to save Danny and send Walker back to the Ghost Zone, they would meet again. Played much straighter when Walker says the same thing.
Aku said this to Samurai Jack all the time. He even lampshaded it once. "We all know what's gonna happen. You'll swing your sword, I'll fly away, and probably say something like, 'I'll be back, Samurai!'"
Barbados Slim in Futurama: Bender's Big Score subverts it. When Hermes gets his body fixed, his wife Labarbara finally dumps Barbados and returns to Hermes. Barbados's final line is, "You haven't seen the last of Barbados Slim. Now goodbye forever!"
Also counts as Leaning on the Fourth Wall as it was unknown at that point if the series would continue past the other three movies, and the writers weren't going to waste any of them on another Barbados/Hermes sub-plot.
After Alfred destroys Dolf's fascist coup in Alfred J. Kwak, Dolf and his associates are forced to flee. Just before he does that, he shows up at Alfred's door, and warns him that he may have won this time, but that one day he'll come back and be powerful again.
At the end of The Venture Bros. episode "What Colour Is Your Cleansuit?", St. Cloud delivers a comically straight-faced version to Billy Quizboy:
You win this time, but mark my words, and mark them well: I will get you, Quizboy.
Played with in The Simpsons episode Brother from Another Series where Sideshow Bob is arrested for a crime he didn't commit and the Cecil the real culprit leans across and says to him: "Tell them they'll live to regret this." Bob screams it out the window, only to realise he's just incriminated himself.
Also, in the episode The Great Louse Detective. Sideshow Bob states this before vanishing away: "We shall meet again old friend, but now I must steal away into the night!"
Used in Captain Planet, when after the titular hero defeated his Evil Twin Captain Pollution, the villain vowed to return before dispersing. At first, this seems like a subversion as the evil rings that summoned him were destroyed in the end, but it was then double subverted as toxic chemicals revived him without the use of the rings.
Skeleton King: This is only the beginning. I'll return more powerful than ever!"
Chiro: And we'll be waiting.
Real Life example/subversion: Asculf, was the last Norse king of Dublin. When the Normans invaded his city he fled with much of the population, only to return with a big army of Vikings. After a savage fight he was defeated and the Normans, in an uncharacteristically merciful move, decided to just exile him. Asculf promply proclaimed he would be back again with a bigger army and recapture his city so they ended up beheading him on the spot.
"I shall return" General MacArthur on the US evacuation from the Philippines in World War II. His words when the US retook the islands: "I have returned". The US government and military actually wanted it changed to "We have returned" before the landings, but MacArthur refused to draw the attention away from himself. The return it the Philippines was itself controversial, with some arguing that they could have been bypassed entirely and that MacArthur insisted on retaking them purely for personal aggrandizement, while others argue that even if the latter part is true we really did owe it to our Philippine allies to liberate them as soon as possible.