The Priestess of the Beast God is the priestess of Cernunnos within the British Lostbelt of Fate/Grand Order.
Profile
Background
The Priestess was a human woman living during 12,000 BC. During the divergent history of the British Lostbelt, the Fairies of the Beginning failed in their task to forge the Holy Sword to stop the rampage of Sefar upon the World. Sefar eventually killed all life upon the Earth and caused the disappearance of all landmasses of the planet, leaving the Priestess as the single remaining human. She was rescued by the Beast God Cernnunos, living upon the Beast God's body along with the six Fairies. When Cernunnos was betrayed and murdered by the Fairies, they tore her apart and used her remains to create the "humans" of Faerie Britain.