The Horned Helmet artifact can be found at West Garrison, in the southern part of Udûn.
Horned Helmet, Third Age, circa 2920
"This helmet was once used by the Outcasts of Udûn for secret rituals. The horns and the leather were taken from the wild kine of Araw, hunted far from Mordor in the fields of Rhûn. It dates from a distant time when these men were prisoners of Gondor, made to toil on the construction of the Towers of the Teeth.
Even now, the Outcasts remain mistrustful of those wearing metal armor. This helmet is a reminder of dark times past, a harbinger of darker times to come and a symbol of the Outcasts determination to never again live under the yoke of slavery."
"You've let me into your homes, you've let me into your lives, and now you bestow upon me your greatest honor.
Though I come from The Black Gate, though I was once known to you as the enemy, I turn my back on Gondor. You people, you Outcasts, as you've been named, are people of honor, people of strength. I have found love within your tribe, but for your tribe as well. And you have taught me of a life I thirsted for, but never knew existed.
Now it is my time to teach you. I will impart to you all my knowledge of war. The Orcs push us, but we will find strength and push back. And this land will be ours!"