An artifact of extraordinary power that belonged to Praecor Loth, the Horn of Praecor Loth was buried in the vault of the dead king on the world of Rhiannon, and the Guardian wasted the lives of many of his men trying to attain it so its power could be harnessed. However, none of them were successful.
The sound produced from the Horn of Praecor Loth has the ability to shatter buildings and turn back armies, making it a mighty weapon on the field of battle. It could only be blown by one strong of lung or via sorcerous means.
The Avatar used the Horn of Praecor Loth to free Castle Britannia from the Blackrock Dome in Ultima Underworld II, but needed to go through a long procedure of capturing an Air Djinn within his own body in order to have any hope of blowing this instrument of destruction.