Karlach's reaction to Wyll getting dragged to the Hells with Mizora if you fail to save her in act 2 is fucking devastating. Let's make it worse! And then better.
The grief-stricken party continues on without Wyll. Mizora still shows up to help in the fight/try and tempt someone into making a pact with her, all while taunting them with the knowledge of Wyll's eternal suffering as a lemure. She knows from spying through Wyll just how much he and Karlach liked each other (even if they didn't act on it) and knows just how to twist the knife with her specifically.
Meanwhile, Wyll is surprised to find he's not been turned into a flesh monster, but is being treated (comparatively) well as Mizora's pet prisoner instead. Except for all the psychological torture about how the party failed to defeat the Absolute or save his father and they all died terrible deaths for nothing and actually probably didn't even care for Wyll anyway, so tragic.
Cue a dramatic emotional reunion and rescue when Karlach busts down the door with the rest of their very much alive party intent on avenging Wyll. And then they fuck tenderly in Mizora's bed or smth while everyone else does a sidequest to fix Karlach's engine idk
+ Feel free to take Wyll's torture situation in a different or more extreme direction as you like, I just care about the mutual presumed dead part
+ Mizora's taunting is what makes one or both of them realize they had feelings for the other
Wyll/Karlach, reunion in Avernus
The grief-stricken party continues on without Wyll. Mizora still shows up to help in the fight/try and tempt someone into making a pact with her, all while taunting them with the knowledge of Wyll's eternal suffering as a lemure. She knows from spying through Wyll just how much he and Karlach liked each other (even if they didn't act on it) and knows just how to twist the knife with her specifically.
Meanwhile, Wyll is surprised to find he's not been turned into a flesh monster, but is being treated (comparatively) well as Mizora's pet prisoner instead. Except for all the psychological torture about how the party failed to defeat the Absolute or save his father and they all died terrible deaths for nothing and actually probably didn't even care for Wyll anyway, so tragic.
Cue a dramatic emotional reunion and rescue when Karlach busts down the door with the rest of their very much alive party intent on avenging Wyll. And then they fuck tenderly in Mizora's bed or smth while everyone else does a sidequest to fix Karlach's engine idk
+ Feel free to take Wyll's torture situation in a different or more extreme direction as you like, I just care about the mutual presumed dead part
+ Mizora's taunting is what makes one or both of them realize they had feelings for the other