Manga Yuuenchi: Baki Gaiden

The story follows a dark place inside the world of the Baki series, the so-called "Amusement Park". The main character is Mumon Katsuragi, older brother of Katsumi. He ventures into a dark place where money is everything and lives are cheap, the "Amusement Park", motivated to find the killer of his mentor and master Taizan Matsumoto. The killer left a few trails behind and with his wits and physical prowess, he's been able to track him down to the ultimate dangerous fighting arena.

(Source: Baki Wiki)