Syravog, Chaos's Form

Flying, shroud
Whenever you cast Syravog, each player exiles
the top five cards of their library. Add {u},{b},{r}, or {g} for each land exiled this way. This
mana doesn't resolve until end of turn.
—Whenever Syravog attacks, each player reveals
the top card of their library and plays it. At the
beginning of that player's endstep, they must
pay mana equal to it's converted mana cost or
lose the game.