Void Siphon

You hold the black ring forward towards a target that you can see within range, and utter the command word. A targeted creature must make a Constitution saving throw. On a failed save, the target takes 3d6 plus your intelligence modifier necrotic damage, or half on a successful save. You heal for the necrotic damage done. If at least two of the  damage dice are "1", the target also takes one point of exhaustion. If all three of the damage dice are "1", the target takes two points of exhaustion.