Hama, the Waterbender

When Hama, the Waterbender enters the battlefield, you add {u} for each island each player controls.
{4}{b}{t}: Exile target card from an opponents graveyard. You may cast it until end of turn
without paying its mana cost.
If there is 5 or more cards exiled in this game at
the end of your turn, transform Hama, the
Waterbender.