Queen Slime

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

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

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

The Gelatin Crystals appear to change color from pink to blue.

Trivia

 * She appears to be a Hardmode counterpart to King Slime.
 * Queen Slime’s AI was created entirely by developer Grox the Great, while the visual effect was created by Yoraiz0r
 * Queen Slime is one of the 2 new bosses added in the 1.4.0.1 update.
 * The Queen Slime is one of 5 bosses with exclusive music, the others being the Empress of Light, Plantera, Duke Fishron and Moon Lord.