Altar of Atoz

Whenever a creature enters the battlefield under
your control put a page counter on the fortified
land.
Fortified land has "{t}, Remove 3 Page counters:
Target creature you control becomes a black
Zombie in addition."
Fortify Swamp {1} ({1}: Attach to target swamp you control. Fortify only as a sorcery. This card enters the battlefield unattached and stays on the battlefield if the
land leaves.)