All creatures get -1/-1 until end of turn for each Swamp you control.
"It's only torture if you're strong enough to survive. Otherwise, it's a simple, gruesome death. I'm happy with either outcome." —Liliana Vess
Official Rulings for Mutilate
7/1/2012 : The effect is based on the number of Swamps you control when Mutilate resolves. Only creatures on the battlefield at that time will be affected. The effect won't change later in the turn, even if the number of Swamps you control changes.