Bellum is an evil life force. Bellum feeds off the life energy of other beings and as well created the Phantom Guardians. With his powers he fed off the Ocean King's powers until he was too weak to fight, as well brought Link, Tetra and Linebeck to the World of the Ocean King when they boarded the Ghost Ship. He resided in the Temple of the Ocean King on Mercay Island where after fully powering the Phantom Sword, Link went down to defeat him. However later, Bellum is able to fuse with the Ghost Ship which then Link has to defeat with the help of Linebeck and the S.S. Linebeck using it's cannon. Although thought to be defeated, Bellum attaches itself unto Linebeck and turns him into the Phantom Guardian-like Bellumbeck which Link defeats to free the pirate and restore the Ocean King.
Source: Wikipedia, "Characters in The Legend of Zelda series," available under the CC-BY-SA License.