"In discussing the SourceBook, you must first discuss the Terminus but when discussing the Terminus it might be useful to understand what it is not. It is not Hell. It does not exist to punish the wicked. The universe does a good enough job of that task on its own.
It is not the land of conquest, a dark realm of power-mad despots who wage war on all who live. Oh, it’s true it has dimension-conquering megalomaniacs in abundance but so does every other corner of the Multiverse in my experience. Conquest and domination are not the Terminus’ purpose.
The Terminus is… an ending. A violent, brutal ending, entropy given swagger and teeth and sent out into creation like a pack of hounds to tear universes into pieces. It began as Edanth, the closest thing to perfection to ever exist wholly within time, but time corrupted it, and as its corruption grew, so did its desire to blight and ruin all things.
Unus once boasted that all things noble began in their realm. Unfortunately, the same perfection has become spoiled, and what the realm now most loves is the corruption of good. It is the darkness that hungers and will not be sated until all light throughout the multiverse has been extinguished, forever." - Rā’s al Ghūl to Dawnguard.
Once this universe was called Edanth, and for a moment, at the beginning of Creation, it was perfect. It was as perfect as the abstract quality of perfection, the platonic ideal of “perfect” embodied in the physical. Every visual detail was beauty, every sound harmonious, every scent pleasing. However, time is by definition change, and if you change perfection, as time must do, it must cease being perfect. When time began in Edanth, the first measurable fraction of a second brought agony to the universe. Unus, the First One, who was the sole being to exist at the moment of Creation, the only one to ever perceive Edanth in its unblemished glory, wept. From his tears were born the Alphians. Unus shaped them into his likeness and named them his brothers and sisters, and was joyous. Perfection had fallen, but love and laughter had taken its place, and who dares to say that fellowship and brotherhood is so much less than a still, silent moment of perfection? Edanth had fallen from perfection, but echoes of the perfect remained, as they do still in all worlds. The Alphians raced and wrestled and played and labored in the near-perfect world beneath the nascent stars. They created songs, which universes have forgotten, that could enlighten beasts. They inscribed poems, for they could not contain their hearts, but loosed them with their tongues in beautiful words. The Alphians were beautiful beyond measure, mighty beyond the dreams of the stumbling gods of later epochs, and they made the wisest spirits of later days look like fools by comparison. They could see everything in creation as universes formed around them but the Alphians found themselves no longer able to confine their love for one another; they wanted to share it with all Creation. Perhaps they should have contained their enthusiasm and allowed existence to unfold and mature on its own. But they wanted to love all of creation, and love makes fools of even the wisest of us.