Royal Gel

The Royal Gel is an Expert Mode accessory from the Treasure Bag when you kill King Slime. When equipped, Slimes become passive and no longer harm you. They'll pass through you, and you won't take damage or knockback. This does not apply to King Slime. This can be used to harvest massive amounts of slimes with minimal effort. However, being able to defeat Expert Mode King Slime may mean that you probably already have the weaponry to able to defeat other slimes.