The Mantle of Joseph of Arimathea
The Dumnonii maintain that Joseph of Arimathea was Mary’s uncle, and that he traded tin throughout the Roman Empire, which would naturally have brought him to Dumnonia. They also maintain that he brought his nephew to the region in his youth, and that after his execution, he sought refuge with his British friends and contacts. They say that Christianity came to Dumnonia with Jesus’s uncle long before Constantine the Great.
In some stories, Joseph evades capture at the hands of the Roman authorities thanks to his miraculous cloak. Sometimes the cloak is outright magical, making him invisible when the Romans might recognize him. Other times the mantle is much more mundane: a simple marker that any Dumnonii could recognize, giving him help at a critical moment.
According to Myrddin, the cloak is currently guarded by a Dumnonian hermit named Petiacos.
“Whoever was under it could not be seen, and he could see everyone.”
Given the ambiguity in the stories, it would seem that the more magical ones came later, as embellishments on the more mundane truth behind this relic. The magic comes from the social support networks that the cloak gives one access to. It is a powerful symbol to the common people throughout Britain, but the Dumnonii most of all. Those networks will help hide you from any enemies, and report to you all manner of things you could never know otherwise.
When you carry the Mantle you can spend a Fate point to learn vital intelligence from Dumnoni allies.