TPK: Should I let it happen? Am I being fair?

[Eltab]

edit: I wrote the following without checking the dates or reading all the way through the thread.
(I think it's still a cool scenario anyways.)

The cultists summon their demon. He is strong but not yet overpowering.
As the PCs break into the room, a cultist activates another McGuffin. The souls of those who the PCs killed float out (recognizable) and swirl around the room.
The demon realizes what the souls are. He starts sucking them in - and is visibly strengthened as each one is consumed.

1) If the PCs stand and gape until the demon is done consuming all the souls, it will be too late: he is overwhelmingly powerful.
2) If the PCs get a move on and blast the demon in the surprise round, they make good progress against him.
Each soul the demon consumes restores damage he has suffered, and boosts his power (stats).
Figure the mechanics out so the PCs have to concentrate on the demon to 'get ahead' but they can take him down eventually. (The cultists might have something to day about that, though)

If the PCs find a way to free the captive souls and help them escape, bonus points: they are sort-of fixing their earlier mistake.

 

[Psikerlord#]

Implement a formal party retreat rule with a cost, if you dont have one already. Then go to town on them. If they dont run, it's on them.

 

[CAFRedblade]

I'm hoping in the next campaign the new characters get to deal with the devastation and destruction left in the wake of the released demon. Small portals letting lose small demons and raising death/ghosts.

If you come across a situation where souls are being consumed again, when the PC's kill, have the soul visibly get drained away towards the ritual/soul gem item, after a few times, perhaps they'll notice.