Seath’s Betrayal

(You may cast a legendary sorcery only if you control
a legendary creature or planeswalker.)
Exile X cards from the top of each opponent's
library. For as long as those cards remain exiled,
you may cast spells from among them. You may
spend mana as though it were mana of any color
to cast those spells.
And Seath the Scaleless betrayed his own, and the
Dragons were no more.