Tentalus

Tentalus is boss creature that is featured in Skyward Sword. When Link tries to go to the control room on the Sandship, Tentalus appears and nearly destroys the whole ship. Link has to fight it to get Nayru's Flame in order to temper the Master Sword.

Strategy
After opening the door to the power room with the boss key, the battle begins. Fi notes that there seems to be a large lifeform is attacking the ship from the outside, Link must make his way to the deck of the Sandship. However, most of the ship has been destroyed and there are large tentacles blocking the way. In order to clear the way, Link must destroy the tentacles with a Skyward Strike. After reaching the deck, the large squid leviathan Tentalus appears. First, Link must avoid Tentalus's tentacles and continue removing them with Skyward Strikes. Once several tentacles have been removed, Tentalus will surface from the waves and attempt to damage Link with its arms. Link must shoot its large eye with an arrow, stunning it. While it is stunned, Link can attack it directly with his sword, and he can also cut off its tentacle "hair" to receive arrows and hearts. After enough damage has been dealt, Tentalus will break part of the deck causing a few barrels to fall towards Link, and also causes a crate to fall from the higher level of the deck. Using the crate, Link must climb up to the upper level, where the battle continues.

Tentalus will continue to attack in much of the same manner and is damaged in the same way. However, instead of attacking with its tentacles from below, it will attack with its toothed tentacle "hair." They must be dealt with by quickly slashing. Once Tentalus has been defeated, the storm clears. Link will be rewarded with a Heart Container and Nayru's Flame allowing his sword to have enhanced dowsing abilities.

Trivia

 * Before going to the outside Fi tells Link to prepare. Since the whole ship was destroyed it is almost impossible to prepare in anyway.
 * Tentalus is a possible reference to Tantalus, an ancient Greek Mythological figure who was doomed to live in a pool of water as a eternal punishment for his wrongdoings.