A large hexagonal room. In in the center is a pool of water, just deep enough to make it difficult terrain. One end is the stairs the PCs will come up; on the far side is the Darkness Engine and the BBEG. The BBEG turns on the Engine and flees up, leaving some demons (I'm thinking Omoxes atm because of the water). The twist is that the Engine can be sabotaged by the PCs if they get to it within a certain amount of time, but if not something is altered in the next enounter (I can't decide what yet). The Engine is supposed to be the keystone to the BBEG's plot to cover the world in darkness (it's a drow).
I guess my main questions atm are:
1) What should be the consequences of failing to stop the Engine? (In this encounter; they can still stop the Engine by defeating the BBEG later).
and
2) How to design the encounter so it is fair to the players in terms of how long they have to deal with the engine whilst being harried by the monsters?