Could the Player take the "Quest" in a completely new direction by the actions he/she is taking?
Say, there is a Side-Quest to banish a Demon, but instead you release him and becomes his master. Now this wasn't just some "Hey Demon become my summon!" but an actual "Doomsday Demon" making it even more threatening than the task previous task at hand, and he's on your leash
Could the player produce some "doom events" themselves? Basically, instead of facing Sarevok, the Bhaalspawn goes off elsewhere on a "X" choice. Choosing to face other player triggered events that is way more a problem than Sarevok. Or stumbling into some dark deep place and accidentally summon Cthulhu (in a side-quest).
Sarevok is just an ant in a much larger scheme after all... no?
Basically, can the Player kind of become the "threat" narratively?