Sewer's Bane

Sewer's Bane is unblockable as long as there are X islands on the battlefield, where
X is the number of +1/+1 counters on Sewer's Bane.
Whenever a creature dies, flip a coin. If you win the flip, put X +1/+1 counters on Sewer's Bane, where X is the creature's power.