The Horn of Truth, also known as Gabriel's Horn or the Horn of Gabriel, is a powerful divine instrument of Heaven that is used by the archangel Gabriel. As stated by its name, the horn causes any one who's hearing range can reach it, to tell the truth uncontrollably, no matter the will they possess.
The use of this horn to Gabriel is unclear, although according to the archangel himself, the horn has many uses, not just simply forcing those who hear it to tell the truth. It was given to him as a means of signalling the angels, in order to rally to his position, and prepare them in times of impending warfare.
However, its greatest ability is for it to resurrect the dead. For it is said that at the end of the Apocalypse, Gabriel will blow the horn once for the souls of those whom have sinned to spend an eternity of punishment in Hell, while the second blow of the horn will resurrect those whom have committed no ill will towards God.
The horn itself is never seen on Gabriel's person, leading many to believe that it is not a physical object, however, other angels state otherwise. Gabriel is only ever seen with the horn during the End of Days, and with statues of him using the instrument.