Vel'Koz, Eye of the Void

When Vel'Koz enters the battlefield, target
opponent reveals the top seven cards of their
library, all creatures that player controls get
-X/-X until end of turn, where X is the number
of nonland cards revealed this way. Draw a card
for each creature sent to the graveyard this way.
That player then shuffles their library.
Sacrifice a creature: Add one mana of any color
of the sacrificed creature.