The princes of Serbia and Bulgaria as well as the East Roman emperor John V Palaiologos
Answer
The princes of Serbia and Bulgaria as well as the East Roman emperor John V Palaiologos paid tribute to Murad I, the Ottoman sultan, following the Battle of Maritsa in 1371. This battle was a major victory for the Ottomans, who went on to conquer much of the Balkans in the following decades.
Unveiling the Legacy: A Quiz on Murad I - The Mighty Sultan of the Ottoman Empire