The Seeking:

Good day! Please help me find the children's book, the names, unfortunately, I do not remember, and the search engine does not provide anything like this. In general, it begins with the fact that his uncle comes to the boy’s house (seemingly, his uncle was called Max). He seems to have come back from the east. The uncle brought a huge box and a strange red cat. In general, the uncle turns out to be absolutely not the one he pretends to be, the boy begins to be chased by the uncle's coat and a cat, which has greatly increased in size. Through the box, the boy gets into another world where there is a river stix, and there is a war between the morning master and the black lord (or something like that). The Lord rides on a two-headed unicorn (the horns of each head are connected). I already start to think that I invented this book as a child) I hope not, because I want to know what happened next.

Roman Kanushkin "Emperor of Shadows"