When Castiel is lead to an abandoned building in an industrial area, he finds it filled with the corpses of angels, and a pulsating sigil on a wall, which soon dies out. After Gabriel reveals himself to be alive to Castiel, he tells him the angel siren that he was hearing was in fact the Horn of Gabriel, and that its purpose is to unite angels by calling them to it. However, Gadreel was using it to trap and kill angels, working his way up north through Utah, collecting the ingredients from the shops of hunters. Castiel eventually uses the Horn to rally a small group of angels to follow him.
- In lore, this horn was owned by Gabriel, and was able to resurrect the dead.
- It's possible when Cas acquired Heaven's weapons the horn could've been in the bundle he flaunted to Raphael.