Problem: Warlock hero is mechanically solid, but it feels thematically wrong.
Instead of someone who messes with demonic powers and can transform into a demon, it feels more like you are playing a demon that transforms into a gnome when slapped. Casting Demonform costs an action, so it's usually bad thing to do in combat. Instead you first transform and then wander around the dungeon in Demonform, looking for trouble. Form is regained on any kill so there is no actual cost to not use it as soon and often as possible (other than time spend casting). Even if you lose it to a spear trap or something, you are not likely to cast it after a combat starts and will propably regain it in said combat anyway.
Goals:



Solution:
Casting Demonform costs 1 MP and is free action.
Optional ideas:

This would encourage Warlock to stay in the fight and maybe go to the next room more speedily. It might be good for theme if the demon-posessed Warlock would be rewarded for bloodlust.

This is a compensation for the Mind point cost. I'm not sure if the warlock needs this buff, but it would give Demonform some more defensive value.
For my own game, I decided to tie these to an item that the Warlock can buy. I also delayed the MP cost to happen when Demonform ends, so that Warlock can make a meaningful choice whether or not to using last MP is worth it.
Clarifications:
Full text for houseruled Demonform card:
This spell may only be cast on yourself. You can make one attack immediately when you cast this spell. For the duration of the spell, roll 1 additional Attack die and ignore all pit traps. The spell ends when you suffer any Amount of damage. When the spell ends for any reason you lose 1 Mind Point. This spell is restored when you reduce a monster's Body Points to 0.
Full text of the new item:
Demon amulet:
Your Demonform spell now also ends at the end of any turn when you did not attack or cast a spell. If your Demonform spell ends because you suffer damage, this amulet prevents 1 damage done to you.
200 gold. Usable only by Warlock.
I know some people have different intepretations, and this might change how Demonform works for you, but this is how I have intepreted the original rules:







