 Complete the following phrase:  The Ottoman Turks under Bayazid I were becoming a much greater threat during Helens rule than during the reigns of Dabiša and Tvrtko I, helped by their decisive victory over the Serbian lord Vuk Branković, whose land had stood between Helens and Bayazids. The Ottoman army that arrived in Bosnia in January 1398, led by Bayazids sons and the subjugated Serbian lord Stefan Lazarević, was larger than those defeated by Helens predecessors 