The Horn of Gabriel Sigil (or, Castiel called it Angel Siren), is a type of trapping spell, that when the sigil is activated will draw any angels within earshot of it to it. Gabriel stated that this spell is used for uniting any angels to the caster. However, Metatron (with his lieutenant, Gadreel) misused the knowledge of this spell to lure any angels and offer the lured angels to join the Metatron's army and kill any angel which refused to join the army.