>>109120581Should be Queen Slime, unless this is a new variant.
It's one of "The Hallow" bosses (a spreading biome separate/counterpart to crimson/corruption) that spawns in hard mode after version 1.1.
The biome is happy fantasy themed with unicorns, fairies, rainbows, etc. (which are all very deadly and kill you).
https://terraria.fandom.com/wiki/The_HallowQueen Slime in particular is a bullet hell that infinitely spawns very aggressive slime adds.
