Ivan IV ascended to the throne in 1547, the realm including parts of The Duchy of Mieczyslaw descended into anarchy in in 1565 with a partition of the Empire into territories controlled by the Tsar and of the Boyars.
The Boyars power was broken by dispossesion, deportations and the redistribution of land to the Dvoryane (a new class of warriors), the entire Empire was thus exposed to awesome terror : liquidations, elimination of whole cities (1570 Novgorod), decline of agriculture, mass flight of peasants despite a decree tying them to the soil.
The Count of Mieczyslaw, appealed to ROME, without response. His reaction was to organize with many lands west of Mieczyslaw and join in a force that was to reach the Kremlin (castle) of Ivan IV in Moscow.
In 1571 Moscow was seized and burned, though no Mieczyslaw men were involved, having been dispatched to supply duties weeks before the final investiture of the city.
A Diet of the szlachta was called again in 1580, no new Count was named, but the position was elevated to Ducal status with letters from the Holy Roman Emperor.
The first Duke of Mieczyslaw, was the former Count: Jedrek.