Sorcery

A mix of Arcanism and binding principals originating in The Pitch Black Forest. Unique in its application of soul energy to directly create and manipulate Celestial quanta constructs. Unlike arcanism, sorcery does not make use of hard light.
Due to their indefinite lifespans and unusually low soul reserves, most Cotd are hesitant to cast sorceries. However, knowledge of shadow tentacles is considered vaguely equivalent to swimming: useful in niche situations and may save your life, but not everyone bothers with it. The tentacles are extremely useful as a (excessively tiring) limb substitute in a pinch.
While most advanced arcanism focuses on maximizing power and size of spells, sorceries instead focus on control and careful manipulations of existing concepts. This is partly due to the differing building blocks- hard light is inherently a very simple and rigid system, while the mediums of sorceries tend to be more flexible. Basic sorcery is essentially just arcanism with a different medium, while more advanced forms start to play on binding concepts.
Higher level sorcery is generally cast by Empty, given the nature of Cotd. Without the massive reserves of energy to draw from, there is no hope to match an archmage in pure power. Other than the Shrouded, sorcerers very rarely make use of solid shadow.
Sorcery often encroaches upon the creation of life, as complex sorcery mediums are arcanism-based replications of Darkness Patterns of Life.

History

Sorcery is a product of a few factors that arcanism lacks:

• Darklight senses allow one to work with projections in mediums that ordinary humans simply cannot comprehend- this isn't just the ability to see masses of homogenous darklight and therefore creation of solid shadow, any person could do so with sufficient study. The understanding of complex darklight structures, along with how darklight intertwines with conventional light and the extrapolation to how dimensions overlap is absolutely crucial to many more advanced mediums, which brush with the space in between planes.

• Higher level arcanists have more rituals to increase their soul flow, giving them more power. Archmages then work with this increased power capacity to create things on a grand scale. Sorcerers are typically Empty by the time they get seriously into their craft, and therefore must maximize what they can do with minimal equipment. Instead of barging through the door, they've got to figure out how to pick the lock.

• A natural link to the Plane of Darkness allows for one to draw certain esoteric effects (ie haunt & puppeteer summoning).

Describing sorcery as a mix of arcanism and binding is not only an accurate descriptor of the practice, but its origins. During the Pitch Black Forest's endless war period, mercenaries from all over the continent flocked there for opportunities. Among them was the northern binders, not quite at the height of their power, but not yet significantly declined below it.
Note that sorcery came about a few centuries before the rise of 'true' hard light arcanism, so it may even be more apt to say that arcanism is a more pure, fundamentally-focused form of sorcery. However, arcanism is far more prevalent throughout the continent, lacking the natural biological barrier of entry. On top of that, its practitioners are far less isolationist.
Although lucrative to outsiders, much of the period saw significant portions of the region's population in poverty, resources hoarded by a select few. Unless one was part of a great warlord's clan (and even then, stability was in no way guaranteed), access to a stable workshop and peaceful working conditions would be rare. These conditions somewhat mirror what gave rise to arcanism in the north, post-cataclysm: unable to utilize outside resources, people turned their curiosity inward and harnessed the magic of their souls. Natives to the region brought the unique perspective of darklight vision, applying binding principles either scavenged by or taught to them, to create a unique flavor of soul magic.
Some instead argue that the roots of sorcery lie more in observation of psychopomps. This isn't entirely incorrect, although it's important to note that the capability to meaningfully observe psychopomps only came about once the practice had developed to some degree. Without the knowledge brought by binders, sorcery would have developed much later, as darklight-seers would have to stumble through much basic theory in a time period where knowledge was hoarded and information regularly destroyed.