Bloodworm

From WoWWiki

A Bloodworm is a creature that has a chance to be produced from targets (that yields XP or honor) the death knight kills from one of their diseases. The bloodworms attack enemies, healing the death knight for the amount of damage they deal for 20 secs or until killed.

The bloodworms heal you for 150% of the damage they cause. ([1] Source)

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