Transform one of your Undead creation into an Undead Monstrosity. While your Undead is transformed, it gains 2d8+4(1d8 per extra spell level) Temporary hit points and if the temporary hit points are depeleted, the monstrosity reverts back to its original form and the spell ends.
The death knight creates a bond with his monstrosity. While the Monstrosity is active, a ghostly chain links the monstrosity to the knight. When an enemy starts its turn on the chain or crosses it for the first time in a turn, it must make a Wisdom saving throw. On a failed save, the creature takes 2d8 necrotic damage. On a successful save, the creature takes half as much damage.
While the bond is active, the monstrosity can't get out of range (30ft) from the death knight
At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d8 for each slot level above 3rd.