Krag, Absorber of Earths

Burrow - Whenever you cast Krag, Absorber of
Earths, you may exile any number of land cards
from your graveyard. For each land card exiled
this way, target player loses that much life.
{3}{b}{b}{b}, Sacrifice a land: Destroy target
permanent. You can only activate this ability
during your precombat main phase or on an
opponent's turn. If you activate this ability
during your precombat main phase, end the
turn.