The Stronghold by Yuriy Kovalyov

7.20

A solar eclipse activates a magic time portal, and a regular schoolboy Vit'ko from the 21st century goes a thousand years back in time. It doesn’t take Vitya long to recognize the legendary bogatyrs in young Oleshko, powerful Ilya and severe Dobrynya. Their life passes in fighting cumans who try to destroy Rusichi at any cost, using both weapons and black magic. Vitya finds himself in the very midst of incredible events, battles with mythical creatures and acquires real courage.