Sol'kanar, Swamp King

Sol'kanar, Swamp King's power and toughness
are each equal to the number of Swamps you
control.
As long as Sol'kanar is untapped, whenever an
opponent casts a spell, that player may pay
{x}, where X is Sol'kanar's power. If they don't,
tap Sol'kanar and gain control of that spell. You
may choose new targets for it.