Gharnef is the creator of the Imhullu spell, making him immune to attacks; however, a magician using the spell Starlight (created by his former master Gotoh) can kill him.

He was once a star pupil of Gotoh, along with Miloah, but due to his lack of a caring heart, he was disqualified to become the inheritor of the Aura spell. Out of jealousy, Gharnef stole the Darksphere, which twisted his soul, and crafted from it the spell Imhullu. Sometime afterwards, he visited an orphanage ravaged by war and used the feelings of despair in Eremiya, the orphanage's bishop, to wipe her memories and bewitch her to put other orphans through brutal training to become assassins. Eventually, he put his plans into motion. He took over Khadein by killing Miloah, revived Medeus and his Dolhr Empire, and manipulated Michalis into killing his father by spreading a rumor that King Osmond was planning to choose Minerva as his successor. He worked with Dolhr and their allies to conquer the world, although he planned to eventually betray them in the end and crush Medeus with the Imhullu spell and the Falchion. He led the attack on Altea (and may have been the one to kill Marth's father Cornelius) in order to gain the Falchion and kidnap Elice, so that he could gain use of the Aum Staff. However, Marth later defeated him and slew Medeus, but Gharnef's soul remained in the Darksphere, allowing his soul to eventually be returned to the land of the living by his heretic followers, albeit without a physical body to inhabit.

(Source: Fire Emblem Wiki)