The Enchanter! (Like from Dungeons)

The Enchanter is an existing illager in Minecraft Dungeons. It doesn't engage in combat itself, but it enchants other hostile mobs, making them stronger. Enchanted mobs would add a fun new challenge to the game and spice up combat! I'd imagine it spawns somewhat rarely wherever other hostile mobs spawn, as well as raids, seeing as they're illagers. They could drop a book or maybe some lapiz upon death.