I don't know what kind of bargain Redcloak would accept, but the way his character arc has been built up, I'd expect him to reverse his final decision from Start of Darkness and try to get one over Xykon.

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Basically, "my brother was right, I should've helped him kill you instead of killing him, and I will kill you like he meant to."

I expect the trigger for this to be related to the Monster in the Dark and Xykon attempting to kill Redcloak when Xykon realizes Redcloak has been playing him.

The other alternative is a final boss switcheroo where Redcloak destroys Xykon and takes over as the main antagonist.