The earliest sources on the Battle of Kosovo, which generally favour the cult of Prince Lazar, do not mention Milos or his assassination of the sultan. The assassination itself is first recorded by Deacon Ignjatije on 9 July 1389, only 12 days after the battle. The assassination of sultan Murad and one of his sons was also mentioned in the instructions of the Venetian Senate issued to Andrea Bembo on 23 July 1389, although Venetians were uncertain if news about the assassination were true. On 1 August 1389 King Tvrtko I of Bosnia (r. 1353-1391) wrote a letter to Trogir to inform its citizens about Ottoman defeat. Victory over the Turks (Latin: ob victoriam de Turcis) was also reported by Coluccio Salutati (died 1406), Chancellor of Florence, in his letter to King Tvrtko, dated 20 October 1389, on behalf of the Florentine Senate. The killer is not named but he is described as one of twelve Christian noblemen who managed to break through the Ottoman ranks:  "Fortunate, most fortunate are those hands of the twelve loyal lords who, having opened their way with the sword and having penetrated the enemy lines and the circle of chained camels, heroically reached the tent of Amurat [Murad] himself. Fortunate above all is that one who so forcefully killed such a strong vojvoda by stabbing him with a sword in the throat and belly. And blessed are all those who gave their lives and blood through the glorious manner of martyrdom as victims of the dead leader over his ugly corpse."  Another Italian account, Mignanelli's 1416 work, asserted that it was Lazar who killed the Ottoman sultan.  The assassin's first appearance in Serbian sources is in the biography of Stefan Lazarevic, Lazar's son, by Constantine the Philosopher, written in the 1440s. The hero, still anonymous, is described as a man of noble birth whom envious tongues had sought to defame before the prince. To prove his loyalty and courage, he left the front line on the pretext of being a deserter, seized the opportunity to stab the sultan to death and was killed himself shortly afterwards. The initial phase of ignominy and its redemption by a courageous plot of slaying the sultan are narrative ingredients which would become essential to the Serbian legend as it evolved in later times.

Answer this question "What was the assassination of the sultan?" by extracting the answer from the text above.
The assassination itself is first recorded by Deacon Ignjatije on 9 July 1389, only 12 days after the battle.