Invasion of Lorwyn // Winnowing Forces

Card text

(As a Siege enters, choose an opponent to protect it. You and others can attack it. When it's defeated, exile it, then cast it transformed.)

When this Siege enters, destroy target non-Elf creature an opponent controls with power X or less, where X is the number of lands you control.

Winnowing Forces's power and toughness are each equal to the number of lands you control.