Sea King

The Sea King is a giant squid boss that appeared during the Second Event. It was discovered by Maple and Sally after Kanade gave them a hint on how to get to it. It's another prank boss, as the only thing that can defeat it is attack the main body, which is hidden deep in the ocean that no player can get to it. Destroying its tentacles will only cause them to regenerate. Maple overcame it by poisoning the entire sea; thus defeating it relatively easily.

In the light novel and manga it contains a skill that allows it to summon fish to attack its enemies which is now in the hand of sally.