There are two different Silver Horns found in Ultima games. The first one has the power to banish daemons; the second one is a gargoyle item. There exists no explanation regarding whether they are related or not.
Silver Horn from Ultima IV
In Ultima IV, the Silver Horn is found by the Stranger on a small island south of Spiritwood. The Shrine of Humility is guarded by daemons in this game, but by using the Silver Horn, the Stranger is able to bypass them and meditate at the Shrine. Although it is possible to reach the shrine without the Silver Horn, it is dangerous and time consuming due to the volume of daemons guarding it.
Gargish Silver Horn
Another Silver Horn appears in the Realm of the Gargoyles in the possession of the gargoyles in Ultima VI. When blown, it summons silver serpents. The horn plays no significant part in the game itself.
In Ultima VII, the horn had been relocated to the Royal Museum in Britain. It is stated in the game that this is the same horn that appeared in Ultima VI, although it has lost the power to summon silver serpents.