Oberon, Emergent Guardian

({t}: Add {u} to your mana pool)
{u}{u}, {t}: The next instant or sorcery spell you cast this turn has split second.(As long as a spell with split second is on the stack, players can't cast spells or activate abilities that aren't mana abilities.)
{4}{u}{u}: Oberon becomes a 6/6 blue Elemental creature with hexproof until end of turn. It's still a land.