The Ur Dragon

Flying, cannot be exiled, die, or targeted
(Power and Toughness is X/X)
Any player may activate the following ability:
{x}{x}: Return The Ur dragon to the command
zone, its controller gets X+1 0/2 Egg creature
tokens with ({1}: Remove an Egg counter),
and (When this creature has zero Egg counters, sacrifice it and put onto the battlefield
a 6/6 Dragon creature token with flying), and
enters the battlefield with X+1 egg counters.