While it is unknown what caused the split between the Seelie and the Unseelie, there are myths and legends which explain the Unseelie's retreat into the Underdark, also called Saekkamata (새까만) in Elysian, and Daimhair in Seelie. For the Seelie fae, these myths claim that the Unseelie were forced out of the Upper Lands due to an inherent criminal nature. The Seelie for the most part do not believe in the true existance of the Unseelie. However, in Unseelie legend, both groups left the Upper Lands due to a catastrophic weather event, which killed majority of their population. In these stories, many fae returned to the Upper Lands when it was safe to do so. However, some stayed in the Underdark, eventually becoming the Unseelie fae.
Magick comes from an organ next to the heart, and is controlled through external organs on the back, vestiges of the wings of their common ancestor with the Seelie fae. These magick organs are highly specialized, with genetic determinants of what kinds of magick an individual might do. The Unseelie call this the geungeoji (Common:invidia venustas).
After years of living underground rendered wings unnecessary, the magick in Unseelie adapted to channel itself through lines, called jul, which sit in the same place on the back as wings, above the shoulder blades. Jul can extend all across the body, but often are more visible on parts of the body related to ones specialty. Along with this, different specialties result in further development of magickal organs in the parts of their body which are used in their magick. Ex: Those with specialties in mind-based magick, such as tricksters, compellers, etc. have further magick organs in their throat. Wolgwang seers, fortune tellers, and others with vision-based magickal specialties have sensors in their eyes.
The eyes of Unseelie fae are highly adapted to their dark underground environment. They have highly sensitive eyes which allow them to see extremely well in low-light environments, but this comes at the price of sensitivity and the risk of blindness in brighter areas, such as on the surface. Their skin is also more sensitive to UV radiation, unsurprisingly. Those few Unseelie who travel to the surface have to wear lots of long-sleeved clothing and long pants and skirts to avoid serious skin damage. They also wear sunglasses or other eye protection to prevent vision impairment.
Skin colors in the Unseelie are determined in the same way as the Seelie, but adaptations to life underground, as well as changes to magick hormones, have altered the structure and expression of melanin and blood. Skin colors among the Unseelie are very often shades of purple, varying between dark, almost black, to very very light purple. These variations are determined by proximity to the surface, similarly to Seelie skin colors. Darker skin tones are found more commonly close to the surface, while those living in areas deeper underground are lighter in skin tone. Genetics are also a main determinate in skin color.
Unseelie hair colors are determined by adapted melanin, and hormones produced by the invidia venustas, as influenced by genetics. Recorded hair colors are less variable than in other species, but include shades of black, white, silver, and blue. Hair color does change as one ages, with older Unseelie fae having whiter hair, or more silver hair than in their youth.