The Rusted Horn artifact can be found at Durthang Keep, in the western part of Udûn.
Rusted Horn, Human, Battle of Dagorlad, Second Age, circa 3430
"This horn heralded the great victory at the end of the Second Age by the Last Alliance at the Battle of Dagorlad. It blew in defiance of the Mouth of Sauron; an assembled army of more than three hundred thousand Orcs; and the Dark Lord himself who ruled over the forces aligned against the Men and Elves.
It sounded the charge on the battle plain and serenaded the fallen dead, those unfortunate souls who would be forever doomed to haunt the Dead Marshes."
"They say the enemy is weak on the left, and that the Dwarves will tear through them there. Maybe then we can leave these filthy marshes and go home.
Yesterday, the enemy made a call for parley. We saw the emissary's party ride out, a terrible black figure atop a black horse and flanked by two huge men similarly clad in black. They were not allowed to ride among our lines amid suspicion they would scout our disposition and report back to their vile master. Instead, a small group of Elves and Men rode out to meet him. We could not hear what was said, but… there will be no peace.
Around the camp there are whispers. They say the emissary's mouth blackened and burned with Sauron's words and that saying them caused the emissary great pain. He promised only slavery and toil - and I am compelled to believe it. I cannot help but think we will never make it into Mordor. Only death awaits us here."