The person who would become Belphegor was the second child of an ancient Osirian Pharaoh. When his brother ascended the Ruby Throne, he entered the priesthood.
His brother grew paranoid in the following years and feared he would be dethroned. The pharaoh had his brother killed and made sure all refernces to him were destroyed.
Over the Millineum, his name was indeed forgotten.
When tomb robbers looted his tomb several hundred years ago, they stole the only items that contained his name. Already a ghost because of his murder, Belphegor found he could take possession of a tomb robber. He attacked to recover his stolen items. Some of the robbers, not realizing their companion was possessed and fearing a double-cross, fled the tomb and took many artifacts with them.
Belphegor now wanders the earth search to reclaim his artifacts and restore his name to public knowledge.
When he possesses someone, he dons his burial robe and mask which he keeps near at all times. These make the host ethereal and allow Belphegor to use his other ghost abilities using the host as a conduit.