The Atmoran Empire was a nation around the Sea of Rhûn which existed from SA 3299 to the end of the Fourth Age.
Though Atmora is also a location in The Elder Scrolls Series, the two are completely unrelated.
"A Child of Númenor..." Edit
In SA 3254, the Númenorean explorer Abrazân (his Quenya name is lost) set out on a long voyage to the east and south. He encountered many cultures, borrowing knowledge and ideas from them.
In 3258, he established a colony on the shore of the Sea of Rhûn; by this point he had persuaded many people from other places to follow him in this pursuit.To accommodate them, he created two languages, one meant as a ceremonial, military, and naming tongue (very similar to Latin), and another which combined many aspects of the people's tongues. It has structure and pluralization like Latin, Japanese prefix and suffix, Nahuatl verbs, Zulu adjectives, Pitjantjatjara nouns and an overall Pitjantjatjara accent which comes with being raised with it. The two languages are known as High Atmoran and Atmoran Common, respectively. All Atmoran military operations are conducted using High Atmoran, as a unique language is more difficult to translate. As such, all equipment, units, and tactics are "code named" in High Atmoran.
Don't expect Aramirtheranger to actually show you Atmoran Common in use. It'll just be in brackets like [these]. The Atmorans speak it, not him.
In 3316, when Ar-Pharazôn demanded that the Atmorans join him in his conquest of Aman, they (primarily being Faithful and people with no real loyalty to the king) flat refused. The king considered destroying the Atmorans for their insurrection, but Sauron urged that the Numenoreans had to go immediately.