The question is, why? Why didn't those Ancient Powers of Good who managed to imprison the Evil Thing simply not destroy it?
In my novel Sword of the Gods, and its sequel Spinner of Lies, this is something I've had to wrestle with. And this is what I've come up with, which works well enough for broader application:
Imprisoning defeated Evil Things and other cosmically powerful creatures is always better than killing them. Why? Because when they're killed, such entities reform/reincarnate somewhere farther down the time stream, but the where and when is impossible to predict.
By imprisoning an Evil Thing, you're guaranteed to restrict its movements in the future (assuming some clueless nazi/D&D adventuring party doesn't wander by years later and let it loose). On the other hand, killing the Evil Thing releases the essence into the multiverse, and that quanta of Evil Thing Energy will find purchase anew, eventually.
That said, a lot of great feedback was generated in comments where I first posted this, which I'm incorporating here as so: Different imprisoned Evil Things should probably have different reasons for escaping execution. Why? Because more options provide more story potential.
Possible reasons why a given defeated Evil Thing is imprisoned instead of dead:
• Death would release the slain Evil Thing to an inevitable reincarnation at an unknown time and place;
• Moral reluctance on the part of the victors;
• Inability for the Evil Thing to actually die (for whatever reason);
• The Evil Thing's potential as a weapon in some other conflict the victors are concerned about;
• The Evil Thing is linked to the Powers of Good so that to slay one slays both;
• The Evil Thing is an actual agent of a Power of Good and thus it is not so much imprisoned as kept caged until needed;
• And finally, some Evil Things are slain (we just don't usually hear about those in stories).
(Of course, if you're not an ancient power of good, but just a really butch mortal with weapons forged for a god, it's more viscerally satisfying to kill an Evil Thing than imprison it. A mortal's short life span means that killing a powerful demonic entity seems to get the job done well enough, so why worry about future generations?)
