Horn of Gabriel Sigil

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 with in earshot of it to it. Gabriel stated that this spell for uniting any angels to the caster. However, Metatron (with his lieutenant, Gadreel) misused the knowledge of this spell to lured any angels and offer the lured angels to join the Metatron's army and kill any angel which refused to join the army.

Procedure
The spell is rather difficult, because it can lured celestial beings like Angels. Additionally, the other ingredients are very rare and belong to rare, odd, and possibly powerful creatures.

A sigil is performed 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.