Well... fuck. Alright, um...
Oh, what about if you have the choice to give/take bandages from the Mummy in a Purity/Corruption style thing, that led you towards a series of fights with the various demons in the Underground? The Mummy would either:
A) Tell the other demons you were nice and the battles will be much easier (say, they attack for less damage or there are fewer fights in total), and at the end, the Mummy doesn't want to fight you and instead lets you pass freely into the last room,
or B) The demons know you're a jerk and the battles will be harder (attack harder, more fights), and at the end, the Mummy is a boss-battle that refuses to 'die' until you use your powers on it to force it into submission