Morpheel

Twilit Aquatic: Morpheel (覚醒多触類 オクタイール, Awoken Many Tentacled Creature: Octaeel) is the third boss found in Twilight Princess, and is located in the Lakebed Temple. At first, it is an anemone- or charybdis-like creature with a single eye that constantly moves through the tentacles and must be extracted via Clawshot. After some time it will shoot out bombfish which you have to throw at it and take them with your clawshot to defuse them. Morpheel erupts out of the ground to revealing an enormous body that is similar in appearance to an eel, or a sea serpent.

Morpheel is a reference to Morpha from Ocarina of Time. Its name is a combination of "morph" and "eel", with "morph" also being the word Morpha's name is based on. Morpheel's tentacles resembles Morpha's ability to extend a tentacle from its watery body. Morpheel can also move its single eye through its many tentacles, in the same way Morpha can move its nucleus throughout its body, and it has to be removed with the Clawshot, Twilight Princess's new hook device.

Strategy
Morpheel's first form is simple to defeat. Equip the Iron Boots, avoid the Bombfish he shoots out at you, then grab his eye with the Clawshot. Attack it immediately. Repeat. After two times, the tentacles will retract. His eye will get swallowed, ending up on his back. Swim above him, using the clawshot to grab his back, latch onto the eye (the Critical Point). Slash away again, and you will be thrown off. Do this a few more times, and he will go down. Then Morpheel will crash onto his side and knock the water out.

Upon defeating Morpheel, Link will receive the third and final Fused Shadow and another Heart Container.

Trivia

 * Morpheel is possibly the largest boss Link has ever fought, along with Molgera, Dark Dragon, Bongo Bongo, Twinmold and Eox.
 * The strategy for defeating Morpheel incorporates the strategies for defeating both Morpha and Molgera.