The Leviathan

The Leviathan is a creature in LEGO Ninjago: Masters of Spinjitzu. It is an enormous sea creature resembling an octopus, apart from having three snake-like eyes at the tip of one tentacle.

The Last Voyage
Prior to the events of the series, the Leviathan was captured by Samukai in order to guard Dr. Julien in his prison. In "The Last Voyage", it investigated the tower while Julien was harboring the Ninja and their allies, but the scientist was able to conceal their presence from the creature.

Later, the Leviathan attacked the Destiny's Bounty when it attempted to escape the island. The Ninja were unable to escape the beast's tentacles, but Zane noticed a great chain binding the Leviathan to the seabed. Diving underwater, the white Ninja used a group of nearby Starteeth to destroy the chain - moments later, the freed Leviathan released the Destiny's Bounty. As both the ship and the creature left the island, Sensei Wu asked Zane how he saved them, to which Zane replied that he had realized that the Leviathan was just as much a prisoner of the island as Dr. Julien had been.

Trivia

 * It resembles a Kraken, a legendary, mythical sea monster that is known to sink ships.