Contract with the Dark Master

Reveal a creature with Ritual from your library,
or command zone. Then you may put the
revealed creature onto your battlefield if you sacrifice creatures whose combined mana value
is equal to or greater than the revealed creature.
If you do not, add the creature to your hand.
If library, then shuffle.
If the revealed creature is your commander, it
enters the battlefield with a Rampage 1 counter
for each creature sacrificed.