Beleth was a demon who was part of an evil trinity (alongside Yomyael and Sytry).
History
Beleth, Yomyael and Sytry were a trinity of demons that had aided the Asiatic people in battle "for as long as men had placed faith in the dark behind stars".
Peter the Great summoned Beleth and the two demons, hoping they would help him in the war against the Swedes in the Great Northern War, but demanded his future sons, his heart and his soul in return.
He was killed during by the lesser demons after the fall of Satan, alongside Astaroth, Orabas, Suriel, Shax, Usiel, Raum, Semyaza, Moloch, Belial, Dagon and Beelzebub.
Lore
Beleth is a spirit of the Ars Goetia and is described as "mighty and terrible king of Hell", who governs over eighty-five legions of demons.