Bayezid I (Ottoman: بايزيد اول, Turkish: Beyazıt, nicknamed Yıldırım (Ottoman: ییلدیرم), "the Thunderbolt", Serbian: Бајазит / Bajazit; 1360, Bursa – March 8, 1403, Akşehir, Turkey) was the Sultan of the Ottoman Empire, then Rûm, from 1389 to 1402. He was the son of Murad I and Valide Sultan Gülçiçek Hatun who was of ethnic Greek descent.
Bayezid ascended to the throne following the death of his father Murad I, who was killed by Serbian nobleman Miloš Obilić during (June 28), or immediately after (June 29), the Battle of Kosovo in 1389. With this battle, Serbia had become a vassal of the Ottoman Empire. A year later, Bayezid took as a wife Princess Olivera Despina, the daughter of Prince Lazar of Serbia, who also lost his life in Kosovo. Bayezid recognized Stefan Lazarević, the son of Lazar, as the new Serbian leader (later despot), with considerable autonomy.
In 1394 Bayezid crossed the Danube river attacking Wallachia, ruled at that time by Mircea the Elder. The Ottomans were superior in number, but on October 10, 1394 (17 May 1395 ?), in the Battle of Rovine, which featured a forested and swampy terrain, the Wallachians won the fierce battle and prevented Bayezid from conquering the country.
In 1394, Bayezid laid siege to Constantinople, the capital of the Byzantine Empire. Anadoluhisarı fortress was built between 1393 and 1394 as part of preparations for the Second Ottoman Siege of Constantinople, which took place in 1395. On the urgings of the Byzantine emperor John V Palaeologus a new crusade was organized to defeat him. This proved unsuccessful: in 1396 the Christian allies, under the leadership of the King of Hungary and future Holy Roman Emperor (in 1410) Sigismund, were defeated in the Battle of Nicopolis. Bayezid built the magnificent Ulu Camii in Bursa, to celebrate this victory.
Thus, the siege of Constantinople continued, lasting until 1401. The Emperor left the city to seek aid. The beleaguered Byzantines had their reprieve when Bayezid fought the Timurid in the East.
In 1400, the Central Asian warlord Timur Lenk (or Tamerlane) had succeeded in rousing the local Turkic beyliks that had been vassals of the Ottomans to join him in his attack on Bayezid. In the fateful Battle of Ankara, on 20 July 1402, Bayezid was captured by Timur. His sons, however, escaped, and later they would start civil war (see also Ottoman Interregnum). Some contemporary reports claimed that Timur kept Bayezid chained in a golden cage as a trophy. Likewise, there are many stories about Bayezid's captivity, including one that describes how Timur used him as a footstool. However, writers from Timur's court reported that Bayezid was treated well, and that Timur even mourned his death. One year later, Bayezid died — some accounts claim that he committed suicide by smashing his head against the iron bars of his cage (the version given by Marlowe; see below). Other accounts claimed that he committed suicide by taking the poison concealed in his ring.
The defeat of Bayezid became a popular subject for later western writers, composers and painters. They revelled in the legend that he was taken by Tamerlane to Samarkand, and embellished it with a cast of characters to create an oriental fantasy that has maintained its appeal. Christopher Marlowe's play Tamburlane the Great was first performed in London in 1587, three years after the formal opening of the English-Ottoman trade relations when William Harborne sailed for Istanbul as agent of the Levant Company. In 1648 there appeared the play Le Gran Tamerlan et Bejezet by Jean Magnon, and in 1725 Handel's Tamerlano was first performed in London; Vivaldi's version of the story, Bajazet, was written in 1735. Magnon had given Bayezid an intriguing wife and daughter; the Handel and Vivaldi renditions included, as well as Tamerlane and Bayezid and his daughter, a prince of Byzantium and a princess of Trebizond (Trabzon) in a passionate and incredible love story. A cycle of paintings in Schloss Eggenberg, near Graz in Austria, translated the theme to a different medium; this was completed in the 1670s shortly before the Ottoman army attacked the Habsburgs in central Europe. Bayezid (spelt Bayazid) is a central character in the Robert E. Howard story Lord of Samarcand. 
Marriages and progeny
Marriages of Bayezid I:
(m. 1372) - Angelina, Princess of Byzantium, whose second husband was Diego González de Contreras, son of Fernán González de Contreras and wife María García de Segovia
(m. 1381) - Daughter of Süleyman Shah of Germiyan
Valide Sultan (1403) Devlet Hatun or Devlet Shah Hatun - Daughter of Yakub Shah of Germiyan. Descendant of Mevlana Celaleddin-i Rumi through his son Sultan Veled's daughter Mutahhara Hatun who was an ancestor of Yakub Shah
Hafsa Hatun - Daughter of Isa Bey of Aydınoğlu
Sultan Hatun - Daughter of Süleyman Shah of Dulkadir
Olivera Despina or Mileva - Daughter of Prince Lazar of Serbia
Maria, Princess of Greece, daughter of János, Count of Hungary, whose second husband was Payo Gómez de Sotomayor
Issue of Bayezid I:
Ertuğrul - son
Musa - son of Angelina
Süleyman, Sultan of Rumelia 1410-1413, murdered - son of Angelina
Emir Süleyman (d. 1411)
Musa Çelebi, Sultan of Rumelia 1410-1413 (d. 1413) - son of Devlet Shah Hatun
Sultan Mehmed I Çelebi (1389-1421)- son of Devlet Hatun
Kasım - son
Isa, Governor of Anatolia - son of Devlet Shah Hatun
Mustafa, Sultan of Rumelia (d. 1401) - son of Devlet Shah Hatun
Erhondu - daughter
Hundi - daughter
Fatma - daughter
A Commando Battalion in Pakistan Army is named Yaldaram Battalion after him.