Harbor Serpent | Magic: The Gathering
Formats Harbor Serpent is Legal in
Official Oracle Text for Harbor Serpent
Harbor Serpent can't attack unless there are five or more Islands on the battlefield.
Official Rulings for Harbor Serpent
8/15/2010 : The second ability checks how many Islands are on the battlefield (regardless of who controls them) only as attackers are declared. Once Harbor Serpent is declared as an attacker, it will continue to attack even if the number of Islands on the battlefield falls below five.