Avestan Textual Sources
Avestan textual sources refer to the two holy lands of the god Ahuramazda from east to west respectively (and in order of priority,) as Hverkana and Verna. They also state that the people of Verna were not originally Iranians nor did they adhere to any Iranian beliefs or religion. Scholars believe that Hverkana refers to the region in the southwest of today's Turkmenistan, and Verna to a mountainous region around Mount Demavend. (A town known as Verne is still located there near Rine).
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 Veramin 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 Sassanid King Kopad defeated the last Gashnaspian king.
The Mazanderanis never compromised with Kopad and he soon left the region, but he placed Zarmehr on the throne in 537 CE. As a native of the region, he became popular. Zarmehr traced his genealogy to Kaveh, the legendary Smith. During the reign of the Zarmehrians many people gradually converted to Zoroastrianism, and the language of the Mazanderanis was somewhat altered. When the Sassanid empire fell, Yezdgerd III escaped to Tabaristan to make use of the Mazanderani's bravery and resistance to repel the Arabs. By his order, AdarVelash (the last Zarmehrian king) ceded the dominion to Ispahbod Gil Jamaspi in 645 CE, while western and Southern Gilan and other parts of Gil's domain merged under the name of Tabaristan. He then chose Amol as capital of United Tabaristan in 647 CE. The dynasty of Gil was known as Gavbareh in Gilan, and as the Dabuyans in eastern Tabaristan. Farrukhan the Great (the fourth king of the Dabuyans), who was crowned after Khorshid I, expanded Tabaristan to eastern parts of today's Turkmenistan and repulsed the Turks around 725 CE. While the Dabuyans were in the Plainy regions, the Sokhrayans governed the mountainous regions. Venday Hormuzd ruled the region for about 50 years until 1034 CE. After 1125 CE, (the year Maziar was assassinated by subterfuge) an increase in conversion to Islam was achieved, not by the Arab Caliphs, but by the Imam's ambassadors. Mazandaranis and Gilaks were one of the first groups of Iranians to convert directly to Shia Islam.