While Tvrtko was thus consolidating his kingdom at the expense of his neighbours, while Hungary was a prey to civil war, while the various princelings of Servia were eternally fighting among themselves, the Turks were ever marching onward. As early as 1375 Marko Kraljević, the hero of Servian popular poetry, had initiated the disastrous policy of calling in Turkish assistance in a quarrel against another Christian prince. Wishing to reconquer Kastoria and other towns in Southern Macedonia and Albania held by the Musacchi family and their ally George I. Balša, he obtained a Turkish contingent for the enterprise, but was defeated by Balša. In 1376 Tvrtko had allied himself with Knez Lazar, who ruled over the Danubian provinces of Servia (the last remnant of the Servian Empire) against Nicholas Altomanović, and continued to remain on good terms with him after Nicholas’s death. He regarded Knez Lazar’s principality as a buffer State between his own dominions and those of the Turks. After the fall of Niš in 1375, and of Sofia in 1382, he gave Lazar assistance, and in 1387 he sent him a contingent which enabled him to cut to pieces a Turkish army of 20,000 men at Pločnik on the Toplica (Old Servia). But the Sultan Murad I. determined to avenge the defeat, and prepared an expedition against Lazar. The latter, seeing himself in great danger, appealed for help from all his neighbours, but the King of Bosnia alone sent him a force, commanded by Vlatko Hranić. The Servian-Bosnian army, under the leadership of Knez Lazar, with Marko Kraljević as chief lieutenant, had its headquarters at Priština, in the plain of Kossovo—a long plateau surrounded by mountains extending from Verisović to Mitrovica. The Turkish army was commanded by the Sultan Murad in person; the right wing was led by his son Bayazet, and the left by his son Yakub. The fight began early on Wednesday, June 15, 1389, and raged all day. For a long time the fortunes of the battle seemed doubtful, and both sides fought with heroic courage. But at last Bayazet succeeded by a sudden attack in throwing the Servian left wing into confusion. At the same time Vuk Branković, whose name has been handed down to the execration of the whole Servian race as a traitor, abandoned the field of battle with all his division. Then Vlatko Hranić and the Bosnian contingent began to give way, and the main body of the Serbs was driven slowly back. Knez Lazar, after fighting like a lion, was killed in the _mêlée_; Murad was mortally wounded in his own tent by the Servian chief Miloš Obilić, who pretended to be a traitor and to have information to give him. He was himself cut down instantly, and then Lazar’s head was brought in by attendants to cheer the dying Sultan, who expired soon after.