Jalhalla

Jalhalla is the immense, obese leader of all Poes and ghosts. It resides in the core of the Earth Temple, and is responsible for the death of the Earth Sage, Laruto. He owes his existence to a small group of about twelve normal poes, and a mask that uses their power to form Jalhalla. This beast must be brought down in order to partly restore power to the Master Sword, by placing Medli in her new position as the Earth Sage. His name is a play on Valhalla, a kind of warrior's heaven in Norse mythology.

He also makes an appearance in Four Swords Adventures during the stage Near the Fields, as the boss of both The Swamp and during the Infiltration of Hyrule Castle.

Strategy
The trick to bringing him down is to reflect light with your Mirror Shield directly onto him, causing him to solidify and fall. You must then lift the giant off the floor and throw him, causing him to roll into one of the purple columns of spikes on the walls. This will cause his body to split into the many Poes for you to dispose of. To make the task easier, you can use the Hurricane Spin technique (if you've learned it) to quicken the job. Repeat this process and destroy all the Poes by any means to destroy Jalhalla. His mask will appear, and dumbfounded that there are no more Poes to use, he will try to escape. A beam of light will stop this attempt by solidifying him, numbing him, and causing him to crash into the floor and shattering. Link is then rewarded with a Heart Container and a more powerful Master Sword.

He is found again in Ganon's Tower later in the game. He is defeated for a second time (in the same fashion) to break the seal to the door leading to Ganondorf.