The ancient artefact is an item dropped by Vorago that can be used to unlock the Bombiboss pet. It can be obtained after using the maul of omens on Vorago in hard mode on all rotations. Once it is used, it will be destroyed and unable to be received again. When the last piece of the outfit is unlocked, it is possible to receive the ancient artefact on the same kill. Claiming the artefact prompts the message, "As you touch the stone, it takes form and you unlock the Bombi pet."