Bellum

Bellum is the final boss of Phantom Hourglass. He can be fought once you reach the thirteenth form the Temple of the Ocean King. His battle has three phases:

Phase One: Bellum, Evil Phantom

Use the Grappling Hook on Bellum's ooze in order to pull its eyes closer to you. Proceed to slash away. After feeling the wrath of Link's sword, Bellum will hop out of the pool and head through the ceiling. Follow him up using the spiral staircase. Equip the bow and circle around the room. Shoot an arrow out at each of Bellum's tentacles. They each have an eye and once they are hit, they will turn purple. Repeat the process for all eyes.

After a scene involving Oshus, Ciela restores her memory and is confirmed to be the Spirit of Time and Courage. Collect the large orb of phantom sphere the fairy emits and use it to upgrade your Phantom Hourglass. Draw the hourglass symbol. Stop time and slash at Bellum's eye. Repeat until the first phase is complete.