Uprising of Konstantin and Fruzhin

Fruzhin, the son of Tsar Ivan Shishman, and his cousin Konstantin, the heir of Ivan Sratsimir, who had emigrated to Hungary and Serbia as the Ottomans subjugated the Balkans, organized an uprising in northern Bulgaria aided by Wallachian ruler Mircea the Elder, Serbian despot Stefan Lazarević and Hungarian monarch Sigismund.

The Ottoman defeat by Timur in the Battle of Ankara of 1402 provided good conditions for an uprising, and the Christian forces crossed the Danube from the north into the Bulgarian lands: Mircea I, accompanied by Konstantin, invaded Dobruja, while Fruzhin together with the Serbian-supported Hungarian army entered the northwestern Bulgarian lands (around Vidin).

According to some sources, it broke out in 1404, but was quickly crushed by one of Bayezid I's sons, Süleyman, with Wallachia nevertheless retaining Dobruja and Stefan Lazarević extending his realm's territory, whereas Konstantin and Fruzhin's efforts were futile.

According to a third group of historians, who base their theory on the note of Doukas that Bulgarian delegates attended the Bursa negotiations between Mehmed I and Manuel II Palaiologos in 1413, the uprising was crushed as late as 1418 alongside several other unrelated rebellions.

He took part in Dan II's attack on Silistra in 1425, then again in 1426 against Vidin that is briefly recovered and put under Fruzhin's rule.