Araknir of the Thunder

Haste, Flying.
When the Araknir of the Thundering web enters in the battelfield, destroy {x} creeatures, where {x} is the colorless mana spend to cast this spell.
Whenever you pay {p/b}, {p/c}, {p/g}, {p/r}, {p/u} or {p/w} you may pay {x}.