If you want to be a human corrupted into a Keeper, perhaps I could help in that regard. Say your human was a strong-willed character in Virens, and I send my Keeper Clotho on another miscellaneous mission there. Your human discovers her and tries to stop her, and she injects him with mutagen. Rather than turning him into some grotesque abomination, his mutations react with his innate magic to transform him into your own design, awakening new abilities within him like the capacity to summon imps and imbue his energy into a Dungeon Heart. However, the transformation leaves him unconscious, and Clotho tosses him into Saploya river, which washes him downriver. Once he reawakens and makes his way to shore, he can start doing his own thing in the new area.
Just an idea.