The Horn of Cathbhadh has the power to summon the spirits of the dead. When Uther attacked the Isle of the Blessed, it was smuggled to safety before the temple fell. Long before the time of the Great Purge, each year at Beltain the High Priestesses would gather at the Great Stones of Nemeton and summon the spirits of their ancestors. Gaius took part in such ceremonies.
The priestesses had to train for years to perform such rites as entering the spirits world is fraught with dangers. The one thing they were schooled never to do was to not look back at the spirit as the veil closed for that would release the spirit into this world.
The Horn of Cathbhadh must be used to reopen the veil between the worlds and force the spirit back into the spirit world. Only the person who summoned the spirit can force it back from this world.