I've said before that if Threnal is Epiced a new quest should be created either as a side quest to the main or the primary focus wherein Coyle went insane after his original adventure and brush with the forces of Xoriat.
In the time since your original meeting with him, he has made some very foolish bargains with some very powerful entities he has no possible way to recompense. His goal is immortality. He has already been granted incredible power by his dealings, but things are about to come to an explosive head as "collections agents" from Devil, Demon, and Xoriat interests arrive to take Coyle to his Reward.
And as much as it might be enjoyable to let them do just that, YOU must slay these agents (sub-bosses) to prevent their forces gaining a foothold in this world.
The final trial comes when it is revealed that Coyle's plan for gaining immortality hinged upon an insanely convoluted scheme whereby he found a way to traverse time and stole the True Sliver of Time from the Gann deposit box months before Tremas did in the Chronoscope and replaced it with a fake. The True Sliver CAN absorb immortal essences AND the powers that were attached to them in life. When you slew the Fiendish agents, you unwittingly started the alignment ritual that will grant Coyle immense power... And True Immortality.
You must stop Coyle now while his powers are fluctuating wildly, with him having at any given moment the powers of a Pit Fiend, a Marilith (including arms and swords), or the... "special" abilities of the Xoriat agent. Let's just say that you slew his mate and offspring and he wasn't too happy about that. Now you've slain him and given his powers to Coyle and he's DELIGHTED about that!
Call it... Immortal Coyle.