Ancient Vows Quest Medallion Piece Missing

The Ancient Vows quests requires me to "Find something that can be placed in the altar", after going into The Titan Grotto and killing the boss I got the Medallion piece, however when returning to the altar the piece is no longer in my inventory. I cannot find it anywhere, resetting the instance and fighting the boss does not drop the medallion. Can anything be done? I'd rather not be locked out of an entire questline.