Ammomancer

At the beginning of your end step, put a Shell counter on Ammomancer, then if there are 3 or more shell counters on this creature, remove 3 shell counters from Ammomancer, gain 1 curse, and create Shelleton, a black 5/5 creature token with Deathtouch and vigilance and "When this creature dies, create a Legendary Shelleton
skull artifact"