Sultan Yildirim Bayezit
He was the 4th sultan in the Ottoman dynasty, born in 1360 AD (761 according to the Islamic calendar). His father was sultan Murad I and mother Gulcicek Hatun. He was well educated and became the governor of Kayseri in 1381. He was a warlike and fierce commander, fought very well in Kosovo against the Crusaders where his father was killed by a Serbian soldier. After the war, he ascended the throne in 1389 AD and soon after captured Rumeli (Thrace) and gained the control of Karaman principality in Anatolia, putting an end to other principalities such as Hamitogullari, Aydinogullari, Saruhanogullari, Germiyanogullari and Menteseogullari, thus establishing a political unity within the Empire for the first time after the Seljuks. In 1391 he besieged Constantinople (modern Istanbul) for 7 months, which later on (1395 and 1397) he put two more sieges and built Anatolian Fortress to cut all sea supports to the Byzantines. In 1394 he captured Thessalonica, Peloponnese (Mora), up to Albania.
Yildirim, meaning Thunderbolt in Turkish, was a nickname he got for the speed of mobilizing his army and attacking the enemy. He reorganized the Janissaries, changing their costumes and regulating their ranks and divisions. All soldiers were paid with regular salaries for the first time. He was also a justice person and liked the conversations with intellectuals. He was religious too, built many mosques and medresses all over the Empire, such as the Grand (Ulu) Mosque in Bursa.
After the Ankara War between Yildirim Bayezid and Tamerlane, Ottomans were defeated and Bayezid became a prisoner in July 1402, soon after he died because of emotional and physical disturbances of this defeat, in March 1403. His son, Mehmed I, became the next sultan.