Serpent Bog

Serpent Bog enters the battlefield tapped.
At the beginning of your end step, if you
control four or more Swamps and Serpent
Bog isn't a creature, you may put three
+1/+1 counters on it. If you do it becomes
a 0/0 black Dragon creature with defender
and flying. It's still a land.
Morass, reasonably safe. Mires, probably fine. Quags, begin to worry me. Don't go near marshland!