Bone is a boss pet acquired by completing the

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Bone
Bone
Pet ID: melvorF:Bone
Source:
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Impending Darkness Event
Drop Chance: Guaranteed after 1 clear
Effect: All enemies have: -1% Damage Reduction
Part of 100% Completion: Yes
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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.