Saw Gerrara, Anarchist

At the beginning of each player's end step, that player puts an ambush counter on target land. Whenever a land with an ambush counter is tapped for mana, its owner takes 1 damage.

When Saw Gerrara attacks, you may exert him. If
you do, put an ambush counter on target land. All players take 1 damage for each land they control
with an ambush counter.