When Linden "interviews" Foul as he manifests in Anele, he repeats this same refrain, but adds something I didn't expect:
Well, I do feel a foreshadowing of whatever revelation is in store in The Last Dark. Surely his deeper purpose consists of more than just opening a can of whoopass on the Creator"But if you remain free, apt, and able to satisfy me, my release is assured. Your attempts to oppose me will secure it. The Arch will be torn asunder, and I will reclaim my rightful place among the eternal heavens. My Enemy will be unable to thwart me."
Cunningly the rank voice added, "There is more, but of my deeper purpose I will not speak."

Anyone have any insights or speculations, given where we stand now?
