Demon Lord Ghirahim is the main antagonist and revealed to be the sword spirit of Demise's sword, given life for the sole purpose of restoring his master to his original form. Though he appears delicate and acts in a playful, flamboyant manner, Ghirahim is a cruel sadist with a dangerous short-fused temper and determination to match. He toys with Link in their first two battles, then battled him in his true form prior to Demise's release. With his mission accomplished, Ghirahim returns to his sword form before being shattered during Demise's intense battle with Link. Many debate that Ghirahim is the evil counterpart of Fi, as they are both sentient manifestations of swords, but on opposite sides.
Source: Wikipedia, "Characters in The Legend of Zelda series," available under the CC-BY-SA License.