|“||No. The Horn of Gabriel... one of Dad's little party favors that I never got around to messing with. Metatron dug it up and started blowing, so I came out of hiding.||”|
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.
The spell is rather difficult, because it can lure celestial beings like Angels. Additionally, the other ingredients are very rare and belong to rare, odd, and possibly powerful creatures.
A sigil is used to perform this spell. Gadreel used various ingredients. The only known ingredients that were revealed for it were very rare and odd: blood, griffin feathers and the bones of a fairy (no specific realm is necessary). However, somehow Ian collected the rare and odd stuffs to performs this spell and sold it secretly.
Once the spell is cast, a loud pulsating ringing which only can be heard by angels will lure nearby angels to the source of the "beacon" (the caster himself).