Petri Dish

The Petri Dish summons a pet Slime when used, which deals no damage. It grants the Pet Slime buff that lasts until the player manually cancels it by pressing/X or X/Square on the buff's icon in the armor section of the inventory screen, dies, or uses the Petri Dish again.

It can be obtained from Gold Chests and as a drop from Shadow Slimes.