The Horn of Joshua is a shofar created by Jewish prophet Joshua. God, eventually touched it under unknown circumstances. The Horn is one of the Hands of God; objects touched by God and imbued with a fraction of his power.
Overview
A man named Noel came into possession of the horn after unearthing it from a desert in Saudi Arabia. At some point of his life, Noel make a deal with Crowley for all the knowledge in the world. After ten years, he realized the gift was not worth it. He became sick. He summoned Crowley so they could trade the horn for the cancellation of Noel's contract.
After Crowley sees the horn, he snaps the contract, but also snaps Noel's neck and takes the horn for himself. He contacts the Winchester brothers in hopes of using the weapon against Lucifer.
Sam and Dean decide to use it against Amara instead.