Illuminant Slimes are Hardmode enemies that spawn in the Underground Hallow biome. They replace normal cave Slimes (Black and Mother Slimes) in most cases. These slimes deal a large amount of damage and have more health than their Cavern counterparts. As suggested in their names, these slimes give off a small amount of light. Illuminant Slimes also leave an after-image like the Chaos Elemental and Illuminant Bat, which appears as a long, pink trail.
Just like many other slimes, Illuminant Slimes become passive around the player when they have the Royal Gelaccessory equipped.