Mustafa Çelebi (1380 – May 1422), also called Mustafa the Impostor ( Turkish: Düzmece Mustafa or Düzme Mustafa), was an Ottoman prince who struggled to gain the throne of the Ottoman Empire in the early 15th century. He was the Sultan of Rumelia twice during January 1419 – 1420 and January 1421–May 1422.
Mustafa was one of the sons of Bayezid I, the Ottoman sultan. His mother was Devletşah Hatun, the daughter of Süleyman Şah of Germiyanids and Mutahhara Abide Hatun bint-i Sultan Veled bin Mawlānā Jalal al-Din Muhammad Rumi. After the Battle of Ankara in which his father Bayezid was defeated by Timurlane, Mustafa as well as Bayezid himself was taken as a prisoner of war. While his four brothers were fighting each other during the Ottoman Interregnum, he was held captive in Samarkand (in modern-day Uzbekistan). After the death of Timurlane, he returned to Anatolia in 1405 and hid himself in the territories of the Turkish beyliks.
After the interregnum, from which his brother sultan Mehmet I had emerged victorious, Mustafa appeared in Rumeli (the European portion of the Ottoman Empire) with the help of Byzantine emperor Manuel II Palaiologos. He also had the support of Mircea I of Wallachia and Cüneyt Bey, the ruler of the Turkish Aydinid beylik. Mustafa asked Mehmet I, who had recently defeated his other claimant brothers, to partition the empire with him. Mehmet refused this request and easily defeated Mustafa's forces in battle. Mustafa took refuge in the Byzantine city of Thessaloniki in 1416. After reaching an agreement with Mehmet, the Byzantine emperor Manuel exiled him to the island of Lemnos.  
After the death of Mehmet I in 1421, Mustafa felt that he could easily defeat his nephew Murat II, Mehmet's son and successor. With the help of the Byzantines, he captured Gelibolu, the fort which controlled the strait of the Dardanelles, and after capturing Edirne, the European capital of the empire, he began ruling Rumeli. Next, he proved that he was indeed Bayezid's son and gained the support of the Ottoman provincial governors in Rumeli. Although Murat sent troops over the strait of Bosphorous to defeat Mustafa, even these troops joined his forces in the Battle of Sazlıdere. Growing overconfident in his abilities, Mustafa decided to cross the Dardanelles and complete his conquest of the Asian side of the empire in Anatolia.
However, in Anatolia, Mihaloğlu (a descendant of Köse Mihal), a partisan of Murat who was very famous in Rumeli, encouraged Mustafa's allies to betray him and support Murat instead. Furthermore, some of Mustafa's allies, notably Cüneyt Bey, abandoned him. Mustafa gave up his hopes to conquer Anatolia and escaped to Rumeli, with Murat's forces in pursuit. To cross the strait of the Dardanelles after Mustafa, Murat asked for the help of Genoan vessels, for which he paid an exorbitant price. Murat's forces soon caught up with Mustafa and captured him.  
Mustafa was sentenced to death and was hanged in 1422. Although inter-dynasty executions were common in the Ottoman dynasty, hanging was not the usual treatment for a dynasty member; more "dignified" execution methods were normally used. It is thought that Murat wanted to send the message that Mustafa was not his genuine uncle (although he was) but an impostor. Thus, contemporary Ottoman historians called him düzmece (English: "fake", "impostor") Mustafa. 
A Turkish pretender claiming to be Mustafa was active on the Venetian side during the Siege of Thessalonica (1422–1430).
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