When Jogaila signed the Union of Krewo and became the King of Poland, he left his unpopular brother Skirgaila as a regent in Lithuania. Vytautas saw this as an opportunity to gain power and started the Lithuanian Civil War (1389–1392). His first military campaign against Vilnius was not successful and had to turn to the Teutonic Knights for the second time. Tautvilas joined him again. Joint forces of Vytautas and the Knights attacked Vilnius in September 1390. According to the chronicles of Wigand of Marburg, Tautvilas was killed while trying to capture the Crooked Castle.