what's he so horrified about

Alternatively for older Mob: buys a cheap apartment and turns out theres this demonic spirit that lives there, but its very tormented and Mob would feel bad for exorcising it right away. So he lets it live in his little complex and treats it like a rowdy dog. Leaves it out some food and some specific chairs to flip and float around the kitchen, maybe even a few old dishes to break if it has to.

Eventually it decides it likes Mob, and basically becomes his spirit familiar.

Mairon relaxing in Angband after a busy day of being horrible experiments. Set either before or during Melkor’s imprisonment (Years of the Trees).