Chernihiv was first mentioned in the Rus'-Byzantine Treaty (907) (as Черниговъ), but real time of establishment is not known and according to the uncovered by archaeological excavations of a settlement which included the artifacts from the Khazar Khaganate, it have existed at least in the 9th century. Towards the end of the 10th century, the city probably had its own rulers. It was there that the Black Grave, one of the largest and earliest royal mounds in Eastern Europe, was excavated back in the 19th century.
In the southern portion of the Kievan Rus' the city was the second by importance and wealth. From the early 11th century it was the seat of powerful Grand Principality of Chernigov, whose rulers at times vied for power with Kievan Grand Princes, and often overthrew them and took the primary seat in Kiev for themselves. The grand principality was the largest in Kievan Rus and included not only the Severian towns but even such remote regions as Murom, Ryazan and Tmutarakan. The golden age of Chernihiv, when the city population peaked at 25,000, lasted until 1239 when the city was sacked by the hordes of Batu Khan, which started a long period of relative obscurity.
Chernigov, from Wikipedia
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