The biggest exception to this seems to be the peoples of Tarush Noptii. The vampires have ruled there for centuries; in many cases, literally the same individual vampires. And the Tarushan people seem to be autochthonous as well, with their own language, unrelated (except by borrowing and other areal features) to any other language in the area. In a way, the Tarushans are like the Basques of the Mezzovian Sea.
The Cavusti used to roam up to the southern shores of the Mezzovian Sea, or at least the Tolosa Bay, before population pressure squeezed them further south to the Cavusti highland plains. Everyone agrees that they were in the area before "modern" humans, but their impact was slight, they never came to the northern shores at all, and as wandering hunter-gatherers, they did little to impact the land. Nevertheless, someone used to live here in prehistoric times, as several isolated tell sites, megalith sites, and other hints of ancient ruins indicate. Dark rumors and myths from the Cavusti and others hint at great and terrible peoples who lived here before the Death god fell from the sky in what is now Tarush Noptii.
There are also strange inscriptions on stelae and other stone tablets, that resist being deciphered, but which appear to belong to languages unrelated to any living. Many of these even the ancient vampires of Tarush Noptii claim to be unable to decipher, and their age is uncertain and clearly very great. Some of them are believed to be outposts of mythical Sasserinna, the land that supposedly occupied the Mezzovian basin before it was flooded with water. Conversely, some other legends credit them with being undersea markers from an underwater civilization that lived in the basin when the water level was much higher.
Little concrete is known about the prehistory of the region, and scholars mourn the almost certain fact that they don't know what they don't know. In the words of Professor Vuissancia de Galdames, "The rocks and earth of our Mezzovian basin have been here for eons uncounted, and the tramp of the feet of the soldiers of numberless empires have trod our soil. Although our knowledge of most of these empires is now lost to us, who can say what ancient legacy yet lurks in the shadows of forgotten dolmens, in the tangled, primeval trees of our vast forests, and under the waves of the blue Mezzovian? And who can say what the nature of such legacy will be; whether treasure to enrich the pockets of its finders, or the state of our scholarship, or perhaps an ancient evil long buried and forgotten, slumbering until such time as it shall plague our poor land yet again?"