Consume Thrall

You attempt to convert the false life of your Undead Thralls into life for yourself and your allies.

When you cast this spell, touch an undead creature which obeys your commands and has at least 1 HP. The target creature must make a Constitution saving throw. A creature created or animated by you makes this save at disadvantage. On a failed save, the creature takes one of its hit dice plus your spellcasting modifier (minimum of 1) as damage. Award the damage as HP to either yourself or an ally you can see. 

On a successful save, no damage is taken, and no HP is awarded. Whether the save was a success or failure, if the creature was not created by you, such as those enthralled by the Command Undead feature, it is no longer friendly to you and will no longer obey your commands.