There was no fantasy in the Soviet Union if by fantasy you mean dungeons and dragons and swords and princesses and so on.

All literary works in the Soviet Union were censored before the publication, and censors were checking these works not only for anti-communist propaganda but also for usefulness for the reader. Roughly speaking, writer had to be able to explan why his novel is useful for the soviet people, how it can teach them to be better citizens. And if you think about fantasy in these terms, it's not very useful, most of fantasy works are more about escapism, and escapism was a big no-no for the Soviet literature and culture. You had to live here and now, and work for the better future.

Second reason is lack of references. Soviet fantasy novels couldn't be published, but western fantasy novels weren't translated. First translation of The Lord of the Rings was published only in 1982, and it was only the first volume, for the next ones soviet readers had to wait several years after that.

(I want to add that it wasn't a first translation, it was the first officially published translation. The first translation of LOTR was made in 1960s, and it was crazy enough because the translator tried to explain in comments all the miracles in the book from science point of view).

Anyway, first fantasy translations appeared on the market at the end of 1980s. And shortly after that Russian fantasy became a thing.

Before that we had almost nothing. There were some books that used fantasy settings. For example, It's hard to be a God by Strugatsky Brothers could be read as a fantasy novel, but it's a sociological sci-fi in fantasy setting — we have two civilizations, one is kinda good and advanced, and another is kinda dark and medieval, and the culture clash between them. It's a good book, but it's not a classic fantasy in any way.

But if you are asking about books with royals as protagonists, there were plenty. But they were no fantasy but historical novels. Alexander Dumas and Maurice Druon were especially popular. If we are speaking about Russian authors, I can give you Valentin Pikul (20 mln books sold).

So you could have royals in the books, but no magic, sorry for that.