A birch fungus, or chaga.

Chaga, or birch fungus (kyar, tsyr, tinder oblique), – a long-term parasitic fungus of the tinder family. Mainly found on the trunks of adult birches, rarely-on alder, Rowan, aspen. For medicinal purposes, only chaga collected from birch trees is used, which is why it is called birch fungus.
Chaga reproduces by spores sprouting on the affected areas of the crust. It is noted that old trees are more susceptible to infection by fungi, as over time the tree decreases the ability to form a wound core, preventing the penetration of spores into the wood.
An important role is also played by growing conditions; thus, trees growing in the mountains are less likely to be affected by fungi-tinder, because the wood they have more dense.