Tentalus

Abyssal Leviathan: Tentalus is the boss of the Sandship, the fifth dungeon in The Legend of Zelda: Skyward Sword. It is encountered in the Lanayru Desert area after it has been restored. It resembles an anthropomorphic sea monster.

Battle
Right after Link inserts the Squid Carving and enters the engine room, Tentalus attacks the ship, with its tentacles blocking the corridors. Link must use a Skyward Strike to sever these tentacles as he makes his way to the deck to face Tentalus. Upon reaching outside, the door collapses, and the battle begins.

Tentalus starts off by diving underwater and attacking Link from below with its tentacles. If one of the tentacles grabs him, he must quickly break free or it will throw him off the ship. To draw Tentalus out, Link must destroy each tentacle with the Bow or a Skyward Strike. Once all the tentacles are gone, Tentalus will rise and assault Link directly by slamming its arms down on him. In order to stun the beast, an Arrow must be shot into its enormous eye, causing Tentalus to fall upon the ship so that Link may strike it with his sword. This is easiest for Link if he fires after Tentalus slams one of its arms. After enough damage, Tentalus will start destroying parts of the ship and Link must rise to the higher part while avoiding rolling barrels to initiate the second phase of the battle.

In this phase of the battle Tentalus will attack Link with the tentacles on its head, which have grown mouths and attempt to bite him. Link must cut the attacking tentacles, then fire an arrow at the eye when it stops to roar and attack it once again. Repeat the strategy to defeat Tentalus.

Etymology
Tentalus is possiblly a portmanteau of "tentacle", referring to its several appendages, and "Tantalus", an ancient Greek Mythological figure who was doomed to live in a pool of water as an eternal punishment for his wrongdoings.