Welk Nor, Underworld Slayer

Flying
Whenever Welk Nor deals combat damage to a
creature, you may exile Welk Nor, and create a
10/10 black Chaos Demon with indestructible, "this
creature is dealt damage in the form of -1/-1
counters", and "whenever this creature leaves the
battlefield, you may return Welk Nor from exile to the
battlefield tapped." Then you may pay 20 life and
create two of those tokens instead.