Sword of Necromancy (5e Equipment)
From D&D Wiki
Weapon (any sword), very rare (requires attunement)
This weapon deals 1d10 extra necrotic damage on a hit while being wielded by a creature that is attuned to it. When a creature is reduced to 0 hit points by an attack that involves this weapon, that creature becomes marked if the wielder of this weapon is attuned to it. If the creature dies while it is marked it comes back to life as a zombie under the control of the creature attuned to this item. If a marked creature regains any hit points it is no longer marked. If there are already 6 zombies that were animated by this weapon, this weapon cannot mark or animate creatures.