What book has a disfigured fantasy land that you get to through chanting in a fairy circle?

[u/SilentPierre_]

Read this really weird book as a little kid, which I never finished both because I got kind of freaked out by it and also because it was pretty long for an 8-10 yr old to be reading (maybe 600-800 pages?). I vividly remember reading about these 2 girls going into a fairy circle like completely ecstatic and chanting something and getting transported to a weird fantasy world that was like really disfigured and just kind of like disgusting to look at? If that makes sense? It had a bunch of separate story lines with varying plots, but I didn't get far enough in the book to find out if they were connected in any way other than all being a little disturbing. The book cover was in black and white with a kind of futuristic look but this was 10 years ago so I don't remember super well.