This Great Moravia!
The first major state that was predominantly West Slavic to emerge in the area of Central Europe, chiefly on what is now the territory of the Czech Republic, Slovakia, Poland (including Silesia), Hungary, and Serbia (Vojvodina).
The only formation preceding it in these territories was Samo's Empire known from between 631 and 658 AD.
Great Moravia was thus the first joint state of the Slavonic tribes that became later known as Czechs and Slovaks and that later formed Czechoslovakia.
And the Great Moravian period is associated with the Marhát Mountain.