Complex sentence: It was located by the right bank of the Donets River between the Bakhmut (Бахмут) and Lugan (Лугань) rivers, which constitutes the territories of present-day Luhansk Oblast and Donetsk Oblast of Ukraine.
Split into simpler sentences: It was located by the right bank of the Donets River between the Bakhmut (Бахмут) and Lugan (Лугань) rivers. This area today constitutes the territories of present-day Luhansk Oblast and Donetsk Oblast of Ukraine.