There were multiple of these growing on the bark of a... beech tree, I believe. In the Netherlands.
Here is another shot showing more members of the group:
At the time I thought it was some type of lichen, but now I think I might be either an Hypoxylon or an Annulohypoxylon from searching for similar images.
I think you are probably right, whatever the specific species it's a death sentence for the tree sadly.
Ooh, I wasn't aware of this, but you are right! From what I can find, the fungi belonging to this class tend to live in the tree passively and only transform into this form once the tree is weak and dying !