Nyx Avatar

When you cast this creature, destroy all creatures
and planeswalkers your opponents control.
Flying, indestructible
Put the top thirteen cards of each player's library
into the graveyard: Until the end of turn, you
may play cards put into the graveyard this way.
You may spend mana as though it were mana of
any type to cast those spells. You may only
activate this ability once each turn and only as a sorcery.