Undying, the Almighty Dirge

When Undying, the Almighty Dirge dies, create Tombstone, a legendary colourless artifact token
with "At the beginning of your upkeep, create X tapped 1/1 black Zombie creature tokens, where
X is the number of creatures your opponents
control."
{b}{g}: Undying, the Almighty Dirge gets +X/+X
and trample until end of turn, where X is the
number of creatures your opponents control.
Activate only once each turn.