The Abyss

A plane of pure shadow and darkness, the Abyss exists beyond Terr'ahna, and is home to the darkest and vilest creatures of all, known as Abyssalons. Some believe that the Abyss is also the source of all Black Mystia, although this is not true.
Over 1,000 years ago, during the Second Age, when the Abyss was first discovered, the Council decided to use this dark plane of existence as a prison of sorts for the evilest of all criminals. This, however, proved to be a mistake, as what the Council failed to understand about the Abyss is that it is a plane that brings forth the darkness of a person's heart and twists them into a creature beyond imagining. This is how the Abyssalons evolved. And this is how the Flow between the Abyss and Terr'ahna began to weaken, causing rifts, called Abyssal Gates, to open between the planes, and allowing the Abyssalons into our world.
The Flow between the planes continues to weaken to this day, and upon the double solar eclipse, when both of Terr'ahna's moons pass between it and the sun, it grows even weaker for a time, causing an outbreak of Abyssal Gates to open across Terr'ahna, filling the world with the nightmares from beyond. Fortunately this celestial event is rare, only occurring once every thousand years, but forever more will the people of Terr'ahna fear the Abyss, more so than even the Undergrave itself.
In the Third Age, 3010, the Sechi Legion general, Shadelon discovered a way to evolve his own being into a being of pure energy, ascending to the Abyss. Since his return from the Abyss, he has gained enormous power, the ability to open Abyssal Gates at will and to control Abyssalon creatures. In Revelations, he reveals his plan to seize control of the Sechi Legion from Raago, the Overlord. In Clash of Light and Darkness, he fights Raago to the death, and kills him, making good on his promise. Shadelon's power continues to grow to immense levels until the final book of first generation, when he discovers his own weakness to Rainbow magic at the hands of Frahna, who subsequently kills him, thus causing the collapse of the Sechi Legion once and for all. At least, for now. . .