Looking for a book I've read years ago (Fantasy)

[Bookling]

I am looking for a book I've read years ago. I can only vaguely remember what it was about, and had no luck with google, etc. I've read it in German, but I am sure that it was also published in English, and possibly even has an English author.
Here is what I remember:

A single, enormous fungal-like or amoeba-like entity, white in color, semi-sentient, consumes everything living it touches, grows like a slime mold across the entire valley/canyon/abyss. It is central to the story. It can control or mimic people it touches/has consumed.
The abyss/valley/canyon is hundreds or thousands of meters deep and difficult to access. It is teeming with life, but dangerous for humanoids. People live around the abyss and view it as life-giving, despite its hostility.
The protagonists descend into the abyss at some point in the story. I faintly remember that the fungus/amoeba/slime-mold like thing was set ablaze.
The story has a dark tone.
The book was read in the 2000s and written before 2010.
It was aimed at young adult or adult readers.

If anyone knows what book I am trying to remember, I would be immensely grateful.

[friendofbooks]

Maybe it's "At the Mountains of Madness" by H.P. Lovecraft, but I'm not really sure about that.

[Bookling]

Thanks for replying! No, unfortunately it's not a work by Lovecraft.