Royal Gel

Royal Gel is an accessory obtained from the Treasure Bag dropped by King Slime in Expert Mode. While equipped, Slimes become passive, and will no longer pursue the player, deal damage on contact or fire projectiles, even if the player deals damage to them first. This applies to all slimes, excluding King Slime and the Spiked Slimes he spawns, though the normal Blue Slimes spawned during the battle remain docile. Minions and NPCs will continue to attack nearby slimes.

Trivia

 * This may be a reference to royal jelly, a substance found in bee hives, fed to queen bees and bee larvae.

History
Gel Royal