Tabaristan

Tabaristan (from Middle Persian: , Tapurstān), also known as Tapuria (land of Tapurs), was the name applied to Mazandaran, a province in northern Iran. Although the natives of the region knew it as Mazandaran, the region was called Tabaristan from the Arab conquests to the Seljuk period.

The Amardians are believed to have been the earliest inhabitants of the region where modern day Mazanderan and Gilan are located. The establishment of the early great kingdom dates back to about the first millennium BCE when the Hyrcanian Kingdom was founded with Sadracarta (somewhere near modern Sari) as its capital. Its extent was so large that for centuries the Caspian Sea was called the Hyrcanian Ocean. The first known dynasty were the Faratatians, who ruled some centuries before Christ. During the rise of the Parthians, many of the Amerdians were forced into exile to the southern slopes of the Elburz mountains known today as Varamin and Garmsar, and the Tabaris (who were then living somewhere between today's Yaneh Sar to the north and Shahrud to the south) replaced them in the region.

During the indigenous Gushnaspian dynasty many of the people adopted Christianity. In 418 CE the Tapurian calendar (similar to the Armenian and Galeshi) was designed and its use implemented. The Gashnaspians ruled the region until 528 CE, when, after a long period of fighting, the Sasanian King Kavadh I defeated the last Gashnaspian king.

...
Wikipedia