Bellum

The Evil Phantom, Bellum is the squid-like,  primary antagonist of Phantom Hourglass.

Characteristics and Background
He is a large, maleficent creature adorned with dark eyes; the main  eyeball exists in the mouth cavity, and each tentacle supports one small  eye as well. Until the conclusion of the fourth dungeon, Link and Linebeck are unaware of his existence. However, once a petrified Tetra is found, Oshus,  revealed to be the Ocean King himself, appears to tell the two about  what happened to her. Many years ago, Bellum appeared and began to consume people's Life Force,  an energy which everything in the world possesses. Bellum attacked Oshus, who naturally fought back. However, his attacks were futile, and he was subsequently was locked away at the bottom of the Temple of the  Ocean King, his Life  Force at the mercy of of his nemesis. Bellum then formulated the Sands of Hours, a crystallized version of Oshus' Life Force, to create monsters that attempted to seal away the  Fairies of Power, Wisdom, and Courage. He also created the Ghost Ship,  spreading rumors of a great treasure within it to lure people there, in  order to obtain further Life Force. Because Bellum is obsessed with obtaining forevermore Life Force, he is attracted to  Tetra because of the great amount of Force within her. The Phantoms, found within the Temple of the Ocean King, are his alter-egos, powerful  and nigh-invincible  like him.

Strategy
Oshus reveals that the only way to slay Bellum is with the legendary Phantom Sword,  a sword which only the blacksmith Zauz knows how to forge. After many struggles, Link obtains the Crimsonine Steel, the Azurine Steel, and the Aquanine Steel needed to forge the Phantom Sword. He then finally penetrates the inner sanctums of the Temple of the Ocean King, where Bellum dwells, and uses the power of the Phantom  Sword, the Phantom Hourglass, and the Ciela-made Phantom Spheres to temporarily defeat it. However, Bellum soon takes a revived Tetra and takes control of the Ghost Ship,  attempting to escape with her. With the combined efforts of both Link and Linebeck, however, the Ghost Ship is destroyed. Bellum, still relentless, makes a large piece of the Ghost Ship drop onto the boat. Oshus, who was still on it, is seemingly killed, and Bellum uses this as an opportunity to grab Link. Linebeck, however, manages to free him using the Phantom Blade, but is soon taken by Bellum instead.

Bellum's Transformation
In a last-ditch effort to defeat Link, Bellum takes control of Linebeck, transforming him into an  immense knight, rightly named Bellumbeck. He then attacks Link, whose attacks are initially useless against Bellum's new form, even with the Phantom Hourglass. Ciela, however, hovers behind Bellum, allowing Link to know when Bellum's weak point,  his eye, is open. Bellum uses one of his tentacles to trap Ciela, though, and continues fighting Link. Link manages to loosen Bellum's grip on Ciela several times throughout the battle, giving him Phantom Spheres to freeze time  once Bellum's eye is open. After a long and difficult battle, Linebeck is freed from Bellum's clutches, and the beast turns to the last of the Sand of Hours. Link absorbs it with the Phantom Sword, and Bellum is defeated.

Alternate Strategies
It is possible to defeat Bellumbeck with only having to freeze time twice,  however, for this to be performed, Link must have collected all of the  Spirit of Power's spirit gems and have completely upgraded himself with  such. Link also must have obtained the Great Spin attack. When Link freezes time, he must use the Great Spin attack on Bellum's eye, while the Spirit of  Power is equipped.

It is also possible to stop Bellumbeck's spin attack, if Link performs one himself. If done correctly, the sword clangs against Bellumbeck's, without having to hit him at all.

Trivia

 * The relationship between Bellum and the Ocean King is a reference to the natural enmity between squids and sperm  whales. Bellum's design is very squid-like whereas the Ocean King is that of a  whale.
 * "Bellum" is the Latin word for war.