Could I run another idea by you about the fairies? (I just thought of it while watching "the fairy scene" on YouTube as a refresher.)
What if my character's body acted more like a hive instead of physically becoming the little monster? Like he coughs it up? And it starts as only one fairy, but the more and more humanity my character loses, the more fairies he's able to regurgitate? The fairies would kind of act on their own, not under a lot of outside influence like they attack foe and ally alike: swarm style. Also, my character's body would be absolutely useless (unconscious or whatever) as long as there was a fairy left loose or alive outside of "the hive." The ultimate risk would be the collective of fairies growing too big and no longer wanting to return to my character's body.
Does that make sense? If it's too weird or too much, I'll drop it.
@my Lalia