The Eye of Set is an artifact from ancient Egypt. It was uncovered in the 1920s in the Nile River valley, but was later thought lost during World War II.
Isis and Osiris are the big egyuptian gods. Set, Isis' brother, kills Osiris. Isis assembles the pieces, and created Horus. She told him to keep Egypt safe from Set. They fight for centuries, and eventually meet in a final contest. A boat race. Each have to ride a boat made of stone to the finish line to win. Horus made a wooden boat made to look like stone. Set sank in his real boat made from the cone of a volcano.
After the boat sank Set pulled it to shore from the depths with him. The people revered him, noting his honesty in the race versus the other god. Much like the Eye of Horus the Eye of Set was created to protect his followers. There were only so many made and they were all made from the stone of the boat he sank in. Most of them were destroyed, but those who possessed the Eye of Set were believed to be protected by him. Even if the stories aren't true...it's a piece of Settite history and MUST belong to his chosen!
The powers of a true Eye of Set are said to be given only to a Follower of Set who imbues the stone with a portion of their own blood. The powers are said to be only defensive in nature, specifically created to protect against the ravages of the Sun on the shadowy minions of Set.