Poseidra, the Atlantean Dragon

① If this card is in your hand/GY: You can tribute 3 Level 3 or lower WATERS; Special Summon it.
② If you do: Return all Spells/Traps on the field to the hand, and if you do, if 3 or more cards were rethrned to the hand this way, all monsters your opponent controls lose 300 ATK for each.