I suppose this doesn't really belong here since this happened while I was DMing a game, but there's no such area for DMs, so I'll put it here. I'm running a 4E D&D game and my players just reached the top of a manor infested by creatures from the Far Realm, a place of madness and Lovecraftian horrors. At the top, they found a rift to the Far Realm, out of which flew a huge, tentacled beast called a Mooncalf. The creature had a few minions with it, but they weren't too big a problem, though they fought downstairs, as the mooncalf fought flying above the roof.
Soon enough, the mooncalf began whooping my players around hard, and eventually only one character was left. Even the companion character, who I brought in to give a little extra aid, was slaughtered. The last character went down. I didn't want him to die; I had no idea what to do in the event of a TPK. Luckily, he made his death save (you must fail three to die) and rose, finishing off the mooncalf (yes, the beast was on its last legs when the character began dying). The minions fled after their master died, and the heroes were raised from the dead to continue the adventure. They never came that close to a TPK so far. We'll see if that trend continues.