Tevarian

Tevarian is a language spoken on the southern islands among their priesthood.
Not much is known about the language but scholars believe that its origins date back to the Ancient Empire and that it is some dialect related to the language that spread with the Empire. Some scholars believe that as the Ancient Empire fell and its forces retreated to their homeland, their language slowly lost its usage among the common people and that the existence of the language on the southern islands now relayed on the priesthood of the islands. However, even that is under pressure from outside forces, especially the influence from the Empire of Dercia the Dercian language is slowly but steadily replacing words within the priesthood as well as among the common people of the islands as more and more Crienia people move to the islands, either as merchants or in the search for a better life.
While it's believed that the Tevarian dialect originally had a written version as well, it slowly began to fall out of use. Then it was replaced by the written system they had adopted from the southern coast of the Mainland leaving the oral language the sole survivor.
No scholars have found a suitable reason why the Tevarian oral language currently only survives among the priesthood, and among the people and priesthood on the islands there are no stories that tell about the early history of the language among their people, they simply just tell about how it arrived with the invaders and when they left, it disappeared within a generation once trade with the mainland picked up again.