In 1572, the throne of the Polish-Lithuanian Commonwealth, at the time the largest and one of the most populous states in Europe, was vacated when King Sigismund II of Poland died without heirs. The Sejm was given the power to elect a new king, and in the Polish-Lithuanian royal election, 1573 chose Henry of France; Henry soon ascended the French throne and forfeited the Polish one by returning to France. Báthory decided to enter into the election; in the meantime he had to defeat another attempt by Bekes to challenge his authority in Transylvania, which he did by defeating Bekes at the Battle of Sinpaul. On 12 December 1575, after an interregnum of roughly one and a half years, primate of Poland Jakub Uchański, representing a pro-Habsburg faction, declared Emperor Maximilian II as the new monarch. However, chancellor Jan Zamoyski and other opponents of Habsburgs persuaded many of the lesser nobility to demand a Piast king, a Polish king. After a heated discussion, it was decided that Anna Jagiellon, sister of the former King Sigismund II Augustus, should be elected King of Poland and marry Stephen Báthory. In January 1576 Báthory passed the mantle of Voivode of Transylvania to his brother Christopher Báthory and departed for Poland. On 1 May 1576 Báthory married Anna and was crowned King of Poland and Grand Duke of Lithuania. After being chosen as king in the Polish-Lithuanian royal election, 1576, Báthory also began using the title of the Prince of Transylvania.
Question: Who challenged Bathory's authority in Transylvania?
Answer: Bekes