Liliana developed a fondness for the moment when roaring becomes squealing.

Mutilate

All creatures get -1/-1 until end of turn for each Swamp you control.

Mystery Booster 2

"You mortals turn to dust in the blink of an eye. I only expedite your fate."

Warhammer 40,000 Commander

Duel Decks Anthology: Garruk vs. Liliana

"It's only torture if you're strong enough to survive. Otherwise, it's a simple, gruesome death. I'm happy either way."
—Liliana Vess

Commander 2014

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.