"Piece" of Celestis
The pendant is said to contain a piece of the Holy City of Celestis. It is a potent defensive device that was capable of protecting the Orici from the weapon on the planet Dakara. It is unknown if it is more powerful than the personal shield emitters developed by the Lanteans. It also appears to possess holographic technology, allowing the leader of the Ori crusade to disguise herself in other forms. (SG1: "The Quest, Part 1", "Counterstrike") (SGA: "Hide and Seek")
It apparently contains a neural interface allowing Adria to control it mentally and is capable of extending its shield to encompass others. When converting a person into a Prior, the pendant glows with light engulfing a Human and turning them into one of the evolved preachers of the Ori. (SG1: "The Quest, Part 1", "The Shroud")
- It's possible that this was Adria using her natural abilities rather than one given by the pendant.
The pendant was given to Adria at an unknown time when she reached adulthood. It protected her from the energy wave generated from the weapon at Dakara which was used to slay her followers on a recently converted planet. It continued to protect her when she laid siege from her Ori warship on the planet which continued to generate the energy waves. Their attempts to kill the Orici failed and she succeeded in eliminating that threat. (SG1: "Counterstrike")
During her quest for Merlin's weapon, she used the pendant to disguise herself as the villager Osric and followed SG-1 in hopes of finding the weapon. However, Dr. Daniel Jackson discovered the fact that she was not who she said she was, at which point she decided to end her deceptions and revealed her true form. At this point, members of SG-1 opened fire on her but their weapon's were ineffective at penetrating the shield generated by the pendant. It was later used to protect both her and SG-1 from a solid base hologram "Dragon" that was protecting Merlin's weapon. (SG1: "The Quest, Part 1")
The pendant was still worn by Adria when Daniel betrayed the Orici in an attempt to use the Sangraal against the Ori in their own galaxy. He lulled the Orici in a false sense of security at which point he knocked her unconscious with an energy blast from his hands. The Pendant was incapable of protecting Adria from Daniel Jackson but he mentioned that it took a lot of power to get past the pendant. (SG1: "The Shroud")