Uldrakiel, the Traitor

Flying, menace
Dragon offering (You can play this card any time
you could play an instant by sacrificing a dragon and
paying the difference in mana costs between this and
the sacrificed dragon. Mana costs includes color.)
Sacrifice another dragon: Search your library for
a non-dragon creature card and put it into exile.
As long this card remains in exile, you may cast it
without paying its mana costs.