Wurm of the Underworld

When Wurm of the Underworld enters the battlefield, put X +1/+1 counters onto it where
X is the number of Zombies on the battlefield.
When Wurm of the Underworld leaves the battlefield, gain life equal to the number of
+1/+1 counters on it.
When Wurm of the Underworld would die, exile
it instead.