How do I make a game where a random person is chosen to be something then others need to kill it?

How do I make a game where a random person is chosen to be something, then the others need to defeat it before it kills them? Also, if time runs out, or the selected player defeats the others, the one selected wins and the round restarts. In short, i'm asking how to I make a game similar to darkness 2, possession, the stalker, and murder mystery.

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Killer = game.Players:GetChildren()[math.random(1,#game.Players:GetChildren())]

You'll have to make the rest of the script, this is not a request site.