Manvydas

Manvydas or Monwid was the eldest son of Gediminas, Grand Duke of Lithuania, and inherited Kernav and Slonim after his fathers death in 1341. Nothing else is known about his life. Matthias of Neuenburg mentioned that two sons of Gediminas perished in the Battle of Strva in February 1348. One was Narimantas and the other is believed to be Manvydas.

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Source: Wikipedia