I keep thinking about this book that I read in the early to mid 2000s. It was a children's book about a couple of kids- boy and girl? Sister and brother maybe?- who somehow get into a parallel fantasy universe and have some trials. I remember a villain who, at the time, reminded me a lot of Capricorn from Inkheart, but more inhuman. The fantasy land I remember being very vibrantly described.
It probably wasn't new when I read it, it wasn't very long but I remember it was a bit of a challenging read for me at the time!
I know this isn't a lot to go off, the biggest stand outs to me are the vibrant fantasy land and the villain who child-me compared to Capricorn.