Illuminant Slime

Illuminant Slimes are Hardmode enemies that spawn in the Underground Hallow biome. They replace normal cave 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.

Tips
An easy way for early hardmode players to defeat these is to grapple to a block above lava, then wait for the slime to jump in, then leaving you to only fend off the Illuminant Bats.The Souls of Light will not burn.