A very common Stock Phrase, usually said by a villain as he's about to fight the hero. It can also be said by the hero if It's Personal.
Other words can be substituted for "die" depending on the situation and the rating level of the work in question. One common variation is "Prepare to be destroyed!"
Surprisingly, nobody explores exactly how you prepare to die. Perhaps you're supposed to do it by borrowing large sums of money and unloading long term stocks. Traditionally, you did it by making peace with God, hence this line's occasional substitutes, "Say your prayers!" or "Prepare to meet your maker!" Still, the person who says it usually never gives them enough time to prepare.
When done against a hero, expect the Big Damn Heroes to swing in at any moment. Sometimes it's the Big Damn Villains instead. A Thwarted Coup de Grâce is sure to follow.
Compare I'm Your Worst Nightmare. Contrast Ineffectual Death Threats. Also, contrast Don't Make Me Destroy You, where the character is actually trying to put off the impending death (Or, at least, he is claiming that). A frequent, if highly unimaginative, Pre Ass Kicking One Liner or Pre-Mortem One-Liner.
Contrast You Have No Chance to Survive, where the character telegraphs his death threats well in advance.
Other phrases that mean the same thing include "and now you die" and "time to die."
"Now tell me, for what purpose did you choose to slaughter all of my people!? Depending on your answer, I'LL SEND YOU TO JOIN GOD!! NO, you don't deserve to stand by God alongside my fallen brothers! YOUR ONLY SOLACE FROM MY WRATH WILL BE DAMNATION!"
A variation used sometimes in InuYasha: "Wash your neck!" (The translation notes explained, as in preparation for execution.)
The Big O: "I will reset the world in my own image with my own will. But before I do that, I'll eliminate everyone who doesn't deserve to exist in my new world. And that includes you filthy scum!" It's not entirely clear whether he was talking about the poor people or the Union, but either way, it was just a little uncalled for.
In One Piece, Luffy is talking to a marine on a Den Den Mushi. He tries to tell them to "Wash your neck and wait", but gets it wrong and instead says "Wash your potatoes".
Ranma ½'s Ryoga brings us the fandom-memetic "Ranma, prepare to die!". Flanderization has us believe that it's his catchphrase, but he really only uses it a couple of times.
The Japanese version of the Punisher in 5 Ronin, in true Inigo Montoya-fashion
You are soldiers of the Daimyo. You razed the house of Akagi. There is nothing more to be said.
Secret Six plays with the trope a bit. Scandal is about to run wild through a group of security guards employed by a slaver. She says that they have 30 seconds left to live—enough to pray or make one last phone call. The guards look to each other... and some do start praying or leaving phone messages, having realized that they had it coming.
Roy Batty: I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion. I watched C-beams glitter in the dark near the Tannhäuser Gate. All those moments will be lost in time... like tears in rain... Time to die.
Emperor Palpatine in the climactic scene of Return of the Jedi: "And now, young Skywalker, you will die."
The Sorcerer's Apprentice has Horvath say "And now, young Dave, I will kill you. Yes, right here in this dreadful bathroom. Quite un-stylish, but you know, these things happen."
Among the hammiest examples is Jeremy Irons spitting clear across the set to say "Now is your time to die!!" in the Dungeons & Dragons movie.
Goldfinger: (laughing) No, Mr. Bond. I expect you to die!
Bond talks Goldfinger into shutting off the laser.
Galaxy Quest When the Thermians are showing a smuggled tape off Sarris' ship to the 'crew' of the Protector:
Thermian: I have told you all I know. If you have any mercy within you please . . . let me die.
Sarris: When I grow tired of the noises you make, you shall DIE!
Lost In Space When Dr. Smith is transformed into a self-proclaimed god and begins to eat the older Will Robinson:
Smith: Time to die . . . son.
From Indy, "Mola Ram, prepare to meet Kali, IN HELL!". Because he prefers to use the Hindu deities.
At the end of The Pebbleandthe Penguin,Drake yells out "Say you prayers, Hubie!'' just right before being crushed by the very boulder he was going to use to kill Hubie.
Sark says this to TRON before their final battle (specifically, he says "Prepare to terminate!").
The Crow: When Eric Draven faces down Top Dollar's entire gang at the club, just before the biggest shootout of the movie.
Eric: You're all going to die.
Invasion USA 1985: Chuck Norris's character said "It's time to die..." to the Big Bad in the past, before kicking him into unconsciousness. This caused the Big Bad to have nightmares and to wake up screaming every night. When the final showdown occurs, Chuck's character just says "It's time...". And then he blasts the Big Bad out of the top story window with a rocket launcher.
The Princess Bride, near the end. Inigo Montoya says this to the six-fingered man, as shown in the page quote.
Paul Tobin's Prepare To Die plays this straight for about ten pages, after which it is massively averted. The hero, Reaver, is at the mercy of his arch-nemesis, the Octagon, who tells him to prepare to die. The hero, surprisingly, agrees, on condition that he gets a month to do so. Astonished, Octagon instead gives him two weeks. And that's just the beginnning...
In the Warrior Cats series, Brokentail shouts "Prepare to die!" at a bunch of rats at the end of a chapter in Yellowfang's Secret when the ShadowClan cats attack them.
Live Action TV
In an episode of Star Trek: The Original Series, the Enterprise is confronted by a powerful alien vessel that announces its intention to destroy our heroes, and goes on to say — "we assume you have a deity... or deities," and politely offers to give the Enterprise crew time to make "whatever preparations" they deem necessary. An unusually explicit example of this trope, where the opponent says "prepare to die" and clearly actually means it. And they're all the scarier for that.
In an episode of Star Trek: The Next Generation, Where Silence Has Lease, the Enterprise-D encounters a similar situation, but the alien claims that between a third to half of the ship's crew will be killed as part of the alien's experiments about death. Picard tries a Take That and starts the self-destruct sequence. Subverted in that the alien's experiments related to monitoring how humans reacted to the imminent threat of death without actually intending to kill them. Double-subverted in that the biggest threat to the ship and the crew in this context was Captain Picard himself, since he threatened to kill the entire crew, as opposed to letting the alien choose which of them would survive.
The ending reveals that it's actually a TRIPLE subversion: Picard knew that the alien was actually interested in how humans react to the threat of imminent death, and calls the alien out on it, and gets the alien to confirm he was right by appearing to him.
The Borgs' well-known stock phrase is a variation, as it implies a form of death, at least on a metaphysical level (the death of the individual).
In Boy Meets World, Eric Matthews acts in a Shakespeare play, performing magnificently so... and then he privately utters the words to Jack, his co-actor and whom he's swordfighting with, "Prepare to die, Jedi Master!"
Horatio Hornblower, "The Even Chance": Before duelling with Hornblower, Jack Simpson, a midshipman of a bully and the most despicable villain, takes some joy to boast, and actually blabs that he attempted to murder Archie Kennedy. (He killed Clayton in another Duel to the Death.)
Simpson: I am going to kill you, Snotty. Just as I killed Clayton. And your little pal Archie.
Well, I knew that snake was my own sweet dad From a worn-out picture that my mother'd had, And I knew that scar on his cheek and his evil eye. He was big and bent and gray and old, And I looked at him and my blood ran cold And I said: "My name is Sue! How do you do!? Now you gonna die!"
While not a terrible a offender as it's successor, Demon's Souls manages to kill the player so many times that will make the casual, inexperienced gamer die over and over. Then again, both games are "love letters" of sorts to Nintendo Hard old school arcade games.
Hazama is quite likely to utter this if a given character pisses him off enough. Count on it happening if Makoto is involved.
(after Makoto breaks his plans in Slight Hope): "Relius was going to take care of you, but shit... I just don't think I can WAIT that long..." (after Makoto almost does it again in Friendship): "Hehehehahahahaha! Let's go, you little bitch! HahahahaHAHAHAHAHAHAAAA!" (after Jin parries a hit on Makoto in Decision): "Now then, if you'd please do me a favor and DIE!"
In The Legend of Spyro: Dawn of the Dragon, Malefor says the line directly before his final showdown with Spyro and Cynder. What makes this note worthy is his general all around character makes it come off as truly threatening. It also helps that regardless of whether or not he wins, the Destroyer might finish them off with the world anyway, so he has a pretty good reason for telling them this.
What makes it more effective is the simple fact that Malefor says this during the only time in the entire trilogy that he gets angrydue to Cynder breaking free of his control thanks to the Power of Love.
The Fallout games in general will have little one-liners like this for the players that want to skip the conversation and get right to the shooting.
In Fallout New Vegas, if you see Caesar on his invitation after accumulating a bad rep with the Legion, he'll tell you that it's all a trap and he's going to have you killed on the spot. Shortly after:
In Dragon Age: Origins when playing as a human noble, you eventually run into Arl Howe, the man responsible for betraying and killing your family at the beginning of the game. He's now the ruler of Denerim and acting as The Dragon to the game's Big Bad. When you try to call for blood rights against him, he laughs in your face about how he's framed your family as traitors to the country, meaning you no longer have any right to vengeance against him. Among several dialogue options, one is to tell him: "I'm going to enjoy slitting your throat." And in the game's next mission you do.
In camp, a drunken Oghren tells the Mabari this line when he thinks that the dog took his pants.
One of the dialogue options for confronting Ser Alrik in Dragon Age II's "Dissent" quest: "Die!"
The bosses of Einhander are a pretty mean lot; most of them say things such as "I'll send you to Hell", or "Welcome! Here you will find only your grave.", with a later one even using this trope almost by the letter. All in German, though.
Pokémon of all series uses them...but of course, Never Say "Die" is nearly always in place, so. It's pretty common in the Ranger games particularly, with things like (major spoilers ahead):
Kincaid: This will be your final lesson. Fufufufu...Drapion! Show them no mercy!
And soon after...
"This will be your final resting place! And for the Pokémon onboard, too!"
Shouted by Flarigan in Aqua Teen Hunger Force during one of the first episodes after Frylock destroys the former's "Rainbow Machine". However, he quickly changes his tune after Frylock launches him into the air.
Rolf: Rolf has had enough of your plat-doodle, elder one. PREPARE YOURSELF FOR A MERCILESS THRASHING!!!
The Transformers has several examples. Galvatron says, "Prepare, Autobots, to DIE IN DARKNESS!" at the beginning of Five Faces of Death Part 5, and Cyclonus says, "Autobots, prepare to die!" in The Rebirth part 2.