I feel the Balor eye tattoo detection bonus is not really fitting with his character and what makes him different, with he being a big burly dude with half a house on a sledgehammer handle and all.
Additionally, i was thinking of giving unique encounters and events a tattoo as well, like the Black Crawler fight, surviving X native assaults, drinking 20 mushroom brews and dropping your wallet elsewhere, Monolith visions, Motion binges, life in the underrail is too
colorful not to immortalize some more bits in you