From Melvor Idle
This page is up to date (v1.2.2).
Pet ID: melvorF:Bone
Source: Impending Darkness Event
Drop Chance: Guaranteed after 1 clear
Effect: All enemies have: -1% Damage Reduction
Part of 100% Completion: Yes

Bone is a boss pet acquired by completing the Impending Darkness Event. Unlike other Boss Pets (excluding Pablo), the chance of receiving Bone is not random. Instead, he is guaranteed to be unlocked after the dungeon is completed.