During Act I, you will stumble upon Wyll, the Blade of the Frontiers, but he holds a dark secret that is later revealed.
It turns out that he has an infernal patron named Mizora, who he has struck a deal with to get the powers that he uses to hunt devils.
There is however a complication if he chooses to spare Karlach, which ends up with him getting punished for breaking his contract with Mizora, resulting in his transformation.
What Happens If Wyll Transforms?
Upon being found by Mizora in Act II, Wyll will be transformed as punishment for not slaying Karlach, who is considered heartless and a target for Wyll.
This results in Wyll going through terrible scorching pain until it is revealed that he now has horns to resemble the very creatures that he hunts.
This is considered as being let off easy though because there are far worse consequences for breaching a contract with a patron.
How To Prevent Wyll’s Transformation?
To prevent Wyll from transforming into a fiend, you will need to comply with what Mizora demands and this is to take Karlach’s life.
There will be multiple opportunities for you to slay Karlach but if you prefer to have her in your party, you will have to live with Wyll’s transformation.
Some have found that killing Karlach and resurrecting her afterward can be a way to bypass this, completing Wyll’s quest to hunt her down and preventing his transformation.
Who is Mizora?
Mizora is a Cambion who serves Zariel, the Archdevil who rules Avernus and continues to send her minions to bring back Karlach.
When it comes to Wyll, Mizora plays a huge part in his story and will often be encountered by the party that he is with.
Aside from pestering Wyll, she also ends up rewarding him for completing certain personal quests, which even involve her being saved.
Verdict
If you prefer to keep Wyll normal, this will require that Karlach is killed one way or the other to prevent his transformation.
You can have Wyll kill Karlach as early as Act I but there are also other opportunities as you progress such as their encounter at your camp.
Recruiting Karlach before Wyll will result in him being found later in Act II, instead of being able to recruit him at the grove, giving you less time to decide on how to proceed with them.