| Expert Mode-Only Content: This information (or parts of it) applies only to Expert mode and Expert mode worlds.
Royal Gel is an accessory obtained from the Treasure Bag dropped by King Slime in Expert Mode. While equipped, Slimes become passive and function similarly to Critters, although they cannot be captured. They 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 Hoppin' Jack, King Slime, and the Spiked Slimes he spawns, though the normal Blue Slimes King Slime spawns remain docile. Minions and NPCs will continue to attack nearby slimes.
- Some unexpected enemies, such as flying Slimers and Gastropods, are considered slimes by the Royal Gel's effects.
- This accessory is particularly valuable during Slime Rain, as it allows the player to kill almost all enemies without worrying about taking damage.
- Royal Gel's name may be inspired by "Royal Jelly", a food substance that bee workers feed to bee larvae to change how they grow, specifically so the larvae can become a queen bee.