Queen Slime is a harder version of King Slime, which can be spawned in the Hallow. Like other bosses, Queen Slime has a phase 2.
Queen Slime has 18000, 28800 and 36720 health.
The only way to spawn Queen slime is to find a Gelatin Crystal and use it on the surface of the Hallow biome.
Queen Slime can spawn Crystal Slimes, Heavenly Slimes and Bouncy Slimes.
When at 50% health, Queen Slime essentially turns into a giant Slimer as she grows wings and begins to fly above the player.