Plane of Darkness

Elemental plane characterized by being filled with 'negative light', which is visually perfectly black. While sight in the human plane is based on the reflection of light, natives to the plane "see" through darklight. Unlike light, its speed is much lower, to the point that its natural spread is perceptible (typical speed ~50 m/s).
The actual mechanisms of sight in darklight are unknown, given that it working like ordinary light's reflections off surfaces makes no sense when compared to the reaction times exhibited.
The plane of darkness is famously considered evil by most human groups. Creatures that live in the plane of darkness tend to have outer layers only visible through darklight, resulting in ordinary light only showing their internal organs and bones. Naturally, this causes great distress in most explorers. Stories about the corrupting effects of darkness can be mostly attributed to a combination of this effect, the effect that pure darkness has on human perception and pattern recognition, and puppeteers. ^{Fucking puppeteers.}
The plane itself is incredibly non-Euclidean, even internally (while others are generally only so when linking between between planes). Areas not heavily tied to the human plane (not directly connected to The Pitch Black Forest etc.) tend to lack gravity. Time tends to flow strangely, especially when nothing (or very rigidly patterned things) is happening ; centuries may pass in a moment. This phenomenon is why entities associated with the plane of darkness do not decay with time- the fundamentals of the plane itself scoff at it. A lot of Darkness touches upon most cultures' ideas of the eldritch and unknown, without quite reaching into the reality-breaking mess that is true Eldritch phenomena.

The Surface Dark and the Deep Dark

The 'surface' Darkness, as it is commonly known, is the area of the plane which commonly interacts with the human plane. It is vaguely the equivalent of a massive mesa, cut off from surroundings. The Deep Dark, or Abyss, is easy to reach (if you're willing to metaphorically fall a few hundred feet), but hard to leave, and orders of magnitude greater in scale and alienness. True monsters lurk there, the sorts of horrors which non-Cotd think of when they think of Darkness, and which the Cotd's rituals were originally meant to seal away.
Surface dark is relatively mundane and logical, simply a different environment for life to grow in. Once reaches the Abyss, things start to become incomprehensible. Iridescent blobs of shifting facial features roam, towering one moment and tiny the next. Mockeries of humanity with smiles larger than their faces flicker in and out of existence. Great, branching serpents of skeleton and shadow burrow through the ground, leaving chasms in their wake. The boundary to reach this land is a one-way cliff. To return, one generally has to make a deal with something that resides within.

History

This phenomenon of 'raised' sections of the plane is a result of an old holy order which performed rituals on pinpricks to Darkness within the continent long ago. The Cotd have inadvertently sealed up the most recent pinprick to darkness in their creation of the Pitch Black Forest, utilizing the ancient order's ritual.