"My name is Torg. You killed my father. Prepare to die."
Translation:torgh 'oH pongwIj'e'. vavwI' DaHoH. yIHeghrupchoH.
When given as a command, -rup is indeed prepare. There is no difference in meaning in English between telling someone Prepare to die! and Be prepared to die!
Prepare to cloak. (ST5)
If you command someone to run, you don't need to say yIqetchoH if they happen to be standing still when you say it. You can just say yIqet. If you command someone to sleep, you don't need to say yIQongchoH just because they happen to be awake; you can say just yIQong (this sentence appears in TKD). If you command someone to focus on a target, you don't have to say DoS yIbuSchoH if they weren't already focusing on the target; you can just say DoS yIbuS (this sentence also appears in TKD).
Likewise, when commanding someone to prepare to die — or, if you like, to be prepared to die, not that it makes a difference — you don't have to say yIHeghrupchoH; you can just say yIHeghrup.
When you command someone just to DO something, you don't need -choH. You only need it when you command someone to change something. The only time -choH is used with an imperative in TKD is with the line yItamchoH Be quiet! (i.e., Become quiet!) This is used to mean "Stop being noisy and change your state to being quiet!"