Rhasst, The Darkin Scythe

Trample, Menace, Haste
Whenever Rhaast, The Darkin Scythe blocks
or is blocked, target player must sacrifice a
permanent with converted mana cost X or
less, where X is equal to Rhaast, The Darkin
Scythe's power. If target player can't sacrifice
a permanent, they take damage equal to
Rhaast, The Darkin Scythe's toughness.