Tentalus

Tentalus aslo known as the Abyssal Leviathan is a huge, one eyed humanoid-like creature with countless tentacles that serves as the Boss of the Sandship in The Legend of Zelda: Skyward Sword.

Battle
When Link tries to access the control room on the Sandship, Tentalus appears and nearly destroys the whole vessel with its massive tentacles. After accessing said room with the Squid Carving, the battle begins. Fi notes that there seems to be a large lifeform attacking the ship from the outside, and Link must make his way to the deck of the Sandship. During his ascent, Tentalus's tentacles burst through the side of the ship, halting his progress. Link uses a Skyward Strike perpendicular to the tentacles orientation to cut them away. The rocking of the ship also dislodges barrels that can oppose Link's progress to the ship's deck.

After climbing to the deck, Link must avoids Tentalus's tentacles and continues to remove them with perpendicular Skyward Strikes or the Bow's Arrows. Once several tentacles have been removed, Tentalus surfaces from the waves and attempts to damage Link with its arms. Link must then shoot its large eye with an Arrow, stunning the monster. While it is stunned, Link must then attack it directly with his Sword. In addition, by cutting off Tentalus's tentacle "hair", Link may receive more Arrows and Hearts. After enough damage has been dealt, Tentalus breaks part of the deck, causing a few barrels to fall towards Link, and a crate also falls from the higher level of the deck. Using the crate, Link must then climbs up to the upper level, where the battle continues.

Tentalus continues 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 attacks with its toothed tentacle "hair", which is repelled by regular Sword slashes.

Once Link defeats Tentalus, the storm clears. Link is then rewarded with a Heart Container and Nayru's Flame, upgrading his Sword with enhanced Dowsing abilities.

Fi's comment
"As you can likely gather from its other title, this "Abyssal Leviathan" is said to have been a tyrant of the ancient seas. Its enormous tentacles can be repelled, but they will regenerate after a short time. My analysis shows that by repelling a number of its aggressive tentacles, the beast will reveal itself. There is a 99% probability that the tentacles that attacked you on the ship, Master, can be attributed to Tentalus. I theorize that using a sacred power will be the most effective means of repelling the tentacles.

Results from my analysis confirm that its tentacles can be severed with a well-aimed Skyward Strike. I theorize that the creature's eye is its weak point, but the probability that you can actually hit the eye with a Skyward Strike is approximately 0%. Instead, I recommend utilizing long-range weapons.

To accurately strike at the creature's eye--its weak spot--a bow will be most effective. You can press the analogue stick to move with your bow drawn. I recommend shooting arrows immediately after dodging attacks launched by Tentalus.

In order to attack Tentalus's weak spot, its eye, you must first deal with the tentacles attempting to latch on to you. My analysis shows you can easily slice the tip of the tentacles with your sword.

Because this monster's tentacles attack incessantly and in large number, it is too dangerous to respond to them one at a time. My analysis indicates it is possible to repel the tentacles more efficiently by concentrating an all-out assault on them with your sword.

When the tentacles are repelled, they may drop hearts and arrows. Should you find yourself in danger because of low hearts or arrows, concentrate on attacking the tentacles rather than fleeing from battle."

Trivia

 * Tentalus is a portmanteau of "tentacle" and Tantalus, an ancient Greek mythological figure who was doomed to live in a pool of water as an eternal punishment for his wrongdoings.