Queen Slime

The Queen Slime is a Hardmode Hallow-themed boss summoned with the Gelatin Crystal.

Spawning
Queen Slime can be spawned by having a player use a Gelatin Crystal. This can be done in the Hallow at any time.

Gelatin Crystal can be found Underground Hallowed mixed with Crystal Shards.

Trivia

 * She appears to be a Hardmode counterpart to King Slime.