Slifer, the Sky Dragon

Summon (Your creatures can help cast this spell.
Each creature you sacrifice while casting this spell
pays for {1} or one mana of the creature's color.)
Split second
When Slifer enters the battlefield, if you didn't
cast it from your hand, sacrifice it at the
beginning of the next end step.
Slifer's power and toughness are each equal to
twice the number of cards in your hand.
Tapped creatures your opponents control get
-5/-0. A creature reduced to 0 power
this way is destroyed.