"I am here... can you see me? Darkness... darkness... no sun will rise... the shadow will consume... are you afraid?" - The Specter King, The Plight of Steel, Chapter 53
The Specter King is the name given by the humans to the primordial demon said to have been created by The One after he was banished to The Depths. He is also called The Black King by some characters, and is referred to as Sul-Serrach by the elves, which translates to Sun Swallower.
The Specter King, according to the stories of Mæro, the first of priests, was a demon formed from the fire and smoke in the Depths by The One. His goal was to punish humanity for turning against the Gods and indirectly influencing the Pure Three in their decision to banish The One from the heavens. Human priests then pleaded with the Gods, asking to be saved from their terrible fate. The Gods, who were not fond of the humans in the first place, decided to give them an impossible task: come together in peace, united under a Savior who could lead them. That Savior would vanquish the Black King and keep humanity from facing destruction.
At the end of The Plight of Steel, it is revealed that according to Adrian Starmane, the prophesized Savior, the Black King does not actually exist, and the world was in no real danger in the first place. In the Point of Chaos, however, Jyr, of the Sunfields, reveals that the Specter King does exist, but not in the form that many think.
According to Jyr, three Northmen journeyed to the Sunfields long ago, eventually stumbling across a primeval entity in a cave. One of the men was possessed and mutilated by the entity, and the others were consumed by it. With a new body and source of energy, the entity, dubbed The Specter King, after the image it incites in the minds of men, was able to emerge from its cave and travel across the world to the ancient Castle Myldrazus in Nightfall. After Adrian engages in a mental battle with it at the end of The Plight of Steel, it flees back to the Sunfields, needing to replenish energy. There, it begins to terrorize the country, consuming humans and animals, as well as trees and plants. Jyr was attempting to escape when he wrecked his ship close to Villamaje, and was captured by the forces of Cantarell Villalon.
Whether Jyr's account is true or not is not revealed.
Those who are tormented by the Specter King claim to hear the question "Are you afraid?" both in their minds, as well as from the mouths of people around them. Images, visions, and nightmares are also known to affect victims, typically manifesting in the form of familiar places, people, or objects. Premonitions are also seen by those affected, and these usually miraculously come true. An account once detailed the torment of an older man who claimed that he saw his wife being consumed by fire. A day later, his house caught flame and his wife was killed under the collapsing roof.
The Specter King has no definite form, as it is an ancient spirit from the depths of the earth, though when glimpsed by a living thing, it takes on a form that is familiar to the mind of the viewer. For humans, it appears as a large humanoid figure, armored in black plating. It's eyes are filled with fire, along with the slits between the metal teeth of its helmet. It is also wreathed in a cloak of dark mist and smoke.
It can be assumed, then, that to other forms of life, it appears as something equally as intimidating. Some believe it appears to animals as a large bear, eyes flame and mouth frothing.
When Adrian Starmane glimpses it at Castle Myldrazus, it appears in his mind as the previously described armored figure, eyes and teeth aflame.
"From this outline, a face began to form: one made of metal, with gaping eye holes that held darkness within. Horns grew from the head: sharp pinnacles and spines of steel etched with words and markings in a strange language, writ in blood and shadow... The face remained in his mind, burning him with its gaze, laughing through teeth of metal." - The Plight of Steel, Chapter 53
It is only deterred from consuming him by the appearance of Archeon's spirit, which can be thought of as a manifestation of Adrian's inner willpower.
The name of Specter King comes from a human description. In the origin religious story, it is a spirit created by The One with the power of divine influence, and so it was dubbed literally, king of specters. No spirit had the power it possessed, and so the humans referred to it as the master of all otherworldly entities. Some time later, it was also dubbed The Black King, also by humans, who believed the entity was a creation of black magic and possessed of malicious intentions. This term is an offshoot from the proper name spoken mostly by the unreligious who see the entity as a force of wickedness instead of a heavenly cleanser. Over time, the name became more common, and entered the vocabulary of the religious as well.
Sul-Serrach, the elven name, was derived from an ancient elven story which told of the darkening of the skies that would occur when the entity would appear. Thus, Sun Swallower became its name. The Elves conform to the beliefs shared by those from the Sunfields that the Specter King is a natural entity instead of a divine creation.