Originally described by Elias Magnus Fries in 1826 as a form of Cladonia rangiferina, subsequent studies on its morphology and chemistry led to its elevation to full species status in 1985.

The lichen is recognised by its dark, ash-grey to black base and its distinctive, uneven branching pattern, often accompanied by small red reproductive structures.

It typically grows in wet, acidic bogs and montane areas, where it contributes to local ecosystem functions and serves as a winter food source for reindeer.

Although C. stygia is widespread in boreal regions, it is considered endangered in some areas, for example in parts of Germany, where habitat loss and ecological changes pose significant threats.

[6] Early lichenologists attributed the blackening (melanisation) of C. stygia's tissues to environmental influences—such as prolonged exposure to stagnant water in bog habitats—rather than recognising it as a taxonomically significant character.

This view, promoted by Rutger Sernander and Heinrich Sandstede in the early 20th century,[7][8] persisted until detailed studies in the 1980s demonstrated that melanisation is a consistent taxonomic trait, correlating with other distinctive features such as the red pycnidial slime.

This suggests that the two species may be in the early stages of speciation—a conclusion that is consistent with their overlapping habitats and ecological differences (C. stygia preferring wetter conditions, and C. rangiferina favouring drier sites).

The terminal and near-terminal branches have a relatively rough surface texture with a felty but loosely fibrous covering, and they typically taper gradually to pointed tips.

[6] One additional identifying feature is the red colour of its reproductive structures (called pycnidia), though this characteristic is rarely visible and can be difficult to observe in the field.

[9] Other similar species include C. arbuscula, C. mitis, and C. stellaris, which can be distinguished from C. stygia by their yellow-green colouration and a negative K− spot test, indicating the presence of usnic acid and absence of atranorin.

Chemical spot tests reveal characteristic reactions: the thallus turns bright red with PD (p-phenylenediamine) and pale yellow with K (potassium hydroxide solution).

[6] The species contains several carotenoids, with β-carotene, β-cryptoxanthin, and lutein epoxide being consistently present across specimens collected from different geographic locations.

[20] Elsewhere in Europe, the species has been confirmed in Andorra, Italy, Portugal, Slovenia, and Turkey, where it is typically found on acid soils in montane and subalpine areas.

[21] Furthermore, although it is primarily found in peatlands, C. stygia also occurs on rock outcroppings in woods (particularly in southern Finland) and in Pinus sylvestris-Cladina woodlands further north, where it typically covers only 1–3% of the ground surface.

It is tolerant of wet conditions – able to survive prolonged periods of inundation during spring and autumn – and in such consistently moist habitats, growth can reach up to 2 cm per year.

[6] In Austrian high moor/bog habitats, C. stygia typically occurs in relatively undisturbed areas dominated by Mountain Pine (Pinus mugo), where it grows alongside characteristic bog vegetation.

Common vascular plant associates include the dwarf shrubs Calluna vulgaris (heather) and Andromeda polifolia (bog rosemary), the sedge Eriophorum vaginatum (tussock cottongrass), and Rhynchospora alba (white beak-sedge).

Other lichens found in these communities include Cladonia pyxidata, C. coniocraea, and C. fimbriata, while typical bryophyte associates are Pleurozium schreberi, Aulacomnium palustre, Sphagnum rubellum, and Leucobryum glaucum.

Individual thalli can reach at least 30 years of age, although older portions may decay over time, and growth rates do not appear to be strongly correlated with climate variables such as temperature or precipitation.

Its distinctive morphology—characterised by hollow, round podetia forming thick, bush-like mats with high water-holding capacity—provides stronger soil insulation compared to both bare ground and other lichen species.

Although its darker colouration might imply a role in solar absorption, research indicates that water retention is the primary driver of its insulating properties, effectively reducing soil temperature fluctuations and mitigating freeze–thaw cycles.

Conservationists have successfully maintained C. stygia populations in managed dune and heathland habitats through topsoil removal (plaggen) and lichen transplantation.