When a Pokémon with Primordial Sea enters the battle or a Pokémon gains the Ability Primordial Sea, the Ability creates heavy rain, a unique variation of rain. In a Rotation Battle, Primordial Sea will not activate if the Pokémon is sent out in a rotated-out position, nor will it activate when that Pokémon is rotated in. In addition, Primordial Sea will not activate if heavy rain is already present on the battlefield.
Once in effect, as long as there is a Pokémon on the field with the Ability Primordial Sea active (even if it is not the Pokémon who created it), the heavy rain will remain in effect unless it is replaced. (If the only Pokémon on the field with Primordial Sea is rotated out or switched out, or has its Ability suppressed or replaced, the heavy rain will dissipate.) The heavy rain can be replaced by Desolate Land or Delta Stream.