Eyghon the Sleepwalker was an ancient possessing demon know at least since the times of the Etruscan civilization, where Eyghon's initiates tattooed themselves with his Mark in order to summon the demon.
In 1975, Eyghon's existance was discovered by two young warlocks in London: Rupert Giles, alias "Ripper", and Ethan Rayne, both members of a group of young sorcerers who practiced magics for pleasure and gain. The members of the group: Giles, Rayne, Randall, Thomas Sutcliffe, Deirdre Page and Philip Henry, tattooed themselves with Eyghon's mark and began to summon him. They would put one of them to sleep, and the rest would Eyghon so he could possess the sleeping participant, resulting in a extraordinary high for the one possessed.
At some point the group lost control of Eyghon and the demon took permanent possession of Randall. The rest tried to exorcise Eyghon but only succeeded in killing Randall. Afterwards, the group broke down and each went their way. However, Eyghon swore to kill the rest for toying with him.
By late 1997, Eyghon had already killed Thomas, and then Deirdre, whose corpse he took possession of in order to hunt down Philip. Eyghon chased Philip all the way to Sunnydale High, where Philip had gone in order to find Giles. However, Eyghon killed Philip before the latter could reach Giles. Afterwards, Eyghon took possession of Philip's body, which was taken to the morgue.
Eyghon awoke in the morgue and then tracked Ethan to the Sunnydale High library through the tattoo in the latter's forearm. Eyghon, however, was incapacitated by Buffy Summers and locked in the library's book cage. Later the same day, Eyghon managed to break out of the cage as Philip's body was about to collapse. Afterwards, he took possession of one unconscious person he could find: Jenny Calendar.
Jenny was taken by Giles to his apartment, where the demon manifested and tried to kill Giles, only to be interrupted by the Slayer. Eyghon, now fully manifested in Jenny's body, escaped.
Eyghon went after Ethan, but did not attack him as Ethan had removed the demon's symbol with acid. Eyghon then turned to Buffy, whom Ethan had tattooed with Eyghon's mark. Eyghon and Buffy fought until interrupted by Giles, who offered himself in place of Buffy. The demon tried to kill Giles until the vampire Angel intervened and began to choke Eyghon. In danger of dying, Eyghon jumped to the closest "dead" body -- that of Angel. However, Eyghon hadn't counted on Angel being a vampire, and had to face the demon within. The two demons fought, and Eyghon was defeated and expelled from Angel's body. Without a host, Eyghon then crumbled to ashes and died for good.
Powers and Abilities
Eyghon was a demon that could manifest in our reality only by possessing a human host, either dead or unconscious. In case of dead host bodies, Eyghon's energy would eventually cause them to collapse into a liquid gooey substance. In case of unconscious hosts, Eyghon would take permanent possession, reversible only through the death of the host. In these cases, Eyghon would manifest physically, causing sever deformations in the host body including facial ridges, blue sores on forehead, face and neck, pointy ears, and glowing eyes. This kind of possession would only be reversed by killing the host, or injuring him/her so the demon would believe himself in mortal danger and escape into another host. Eyghon was also capable of establishing a psychic link with the bearers of his mark and in between them, allowing the demon to track them down and cause them to experience vivid, flash-like visions during their sleep. Finally, Eyghon manifested telekinetic powers.