The Battle of Kosovo and the martyrdom of Sultan Murad I
The Ottomans broke the Balkan Crusader alliance led by the Serb prince Lazar on the plain of Kosovo. Sultan Murad I was martyred on the battlefield, and the victory anchored the Ottomans in the Balkans for centuries.
The Serb prince Lazar assembled an alliance of Serbs, Bosnians and others to drive the Ottomans out of Europe, so Sultan Murad I crossed with his army, and the two hosts met on the plain of Kosovo ("Kosovo Polje") on 15 Shaʿban 791 AH (June 1389).
A ferocious battle raged in which the alliance was broken and Lazar was killed. While Sultan Murad was surveying the battlefield, a wounded Serb stabbed him a fatal blow — the first Ottoman sultan to be martyred on the field.
His son Bayezid "the Thunderbolt" assumed rule immediately upon his martyrdom and completed the victory. The battle opened the Balkans to the Ottoman state for five centuries, and Kosovo has remained a Muslim-majority land from that day to ours.
Note — differing reports on the date: It occurred on 15 June 1389, corresponding in conversion to 15 Shaʿban 791 AH according to the well-known view, with slight differences in some references.