Gabriel's Horn is an unseen artifact that is housed inside The Library.
Powers 
Library display card for the artefact- made by user magicexpert28
[]
Library display card for the artefact- made by user magicexpert28
As it was never seen on the series, its powers are unknown.
Real Life Counterpart[]
Gabriel's Horn is a horn-shaped mathmatical object studied since the 18th Century. It has finite volume, but infinite surface area.