So, this post is after reading all of the Cycle except Nightworld and nearly all of the rest of the Secret History.
It's mentioned repeatedly that The Otherness despises humanity, and Rasalom devours sadness and despair. I think there was one mention of Rasalom being unsure of The Otherness because his power was growing slowly right there, but one thing hit me: if The Otherness gains sustenance and glee from sadness and despair, what's to stop it from taking away Rasalom's power after he attains total global domination.
Imagine this line of thought: Rasalom conquers all of Earth making The Ally lose all interest. He controls Earth completely, then in a final act of conquest, The Otherness removes his power. Imagine the pain, fear, sadness, hate, and utter despair Rasalom would feel after millenia of working for this spot only to have no way to maintain it. I don't think any despair could ever compare to that.
It was a random thought I had while idling away for Nightworld's rerelease.