Rorik of Dorestad is based on this personage - https://en.wikipedia.org/wiki/Rorik_of_Dorestad ; as there have suggestion he is also Rurik who is alleged to be the founder of Kievan Rus (this country actually have nothing with now-today Russia, whose origin is from Moscow).
And the Battle of Kalka.... it was an overwhelming Mongol victory... some months after that battle the Mongols suffered great defeat from the Volga Bulgars https://en.wikipedia.org/wiki/Battle_of_Samara_Bend which is nicknamed "Ram Battle", and it is the only defeat of the Mongols during Genghis Khan time.
P.S. Please do not compare with some trash bullshit isekai manhwas like Solo Leveling...