A necklace featured in Fate/stay night. It is a family heirloom of the Tohsaka family which was used by Tohsaka Rin as a catalyst to summon her servant, Archer. She later used its remaining magical energy near the start of the Grail War to revive Emiya Shirou after he was killed by Lancer, losing it in the process. Archer later goes back and retrieves it for her.
During Fate/Unlimited Blade Works, Shirou reveals to Rin that he still has the pendant after his revival, meaning that the one Rin has is a duplicate. She realizes that this is because Archer has his own copy of the pendant he brought with him when summoned, because he held onto it until the moment he died. This revelation makes Rin realize Archer's true name.
