Eyghon, also called the Sleepwalker, can only exist in this reality by possessing an unconscious host. Temporary possession by the demon imbues the host with a euphoric feeling of power. However, unless the proper rituals are observed, the possession is permanent and Eyghon will be born from within the host.

Once called, Eyghon can also take possession of the dead, but it's demonic energy soon disintegrates the host, and it must jump to the nearest dead or unconscious person to continue living. Many ancient sects used to induce possession for bacchanals and orgies.

When Rupert Giles was a young man, he rebelled against his duty as Watcher and joined a group of five individuals to create a cult which worshipped Eyghon. One of them would go into a deep sleep and the others would summon him. They all loved the euphoric feeling it would give them. They continued with their worship until one of the members, Randall, lost control and Eyghon took him hole. They tried to exorcise the demon, but it killed him. Actually, they killed him to stop Eyghon. They succeeded in banishing the demon though.

But the first thing it did when it got back to this reality was to start hunting the rest of the members of the cult. It killed Tomas Sutcliffe and Deidre Page, then it took possession of Deidre's dead body and hunted down Phillip Henry. It found Phillip at the Sunnydale Highschool's doorstep and killed him there. But it jumped bodies once more and took possession of Philip and came after Ethan who had taken refuge in the Library. Buffy managed to stop Philip and lock him in the cage. After Giles arrived Philip broke free of the cage, knocking Jenny Calender out at the same time. Then he disintegrated. No one realized that Eyghon had jumped into Jenny's body until it was almost too late.

Ethan kidnapped Buffy and tied her down so he could tatto her with the Mark of Eyghon. He then burned his own tattoo off with Sulphuric Acid and awaited Jenny to come and kill Buffy. Giles arrove at the same time and a fight ensued as Buffy freed herself from her constraints. Just as it seemed that Jenny/Eyghon was going to kill Giles, Angel burst in and started to strangle Jenny. This action made Eyghon fear for it's life and jump from Jenny's body into the nearest dead one: Angel. But Angel was used to fighting a demon inside him and he managed to thrust Eyghon out of his body. Once Eyghon was freed without a host, he disintegrated into nothing and died.