The Dark One's Mind

The Dark One is a dormant deity, put to sleep by the creator Daeglyses after a quarrel with the Light One nearly saw the the ruin of everything. The Dark One sleeps, but far from peacefully. In their mind, they imagine a universe where their philosophy is fulfilled and enacted without resistance from the preserving force of the Light One. This lair of destruction simultaneously exists and does not exist; it is within the mind of a god so powerful that its dreams have substance.
The easiest way to understand it is as a parrallel reality, where the Light One does not exist.
The Dark One believes that without destruction there can not be creation, and that without evil and challenge there can never be good and triumph. The Dark One's Mind is rife with all manner of destructive forces and entities, dangers and terrors, and somehow... people. Whole cities and populations of people have existed within the Dark One's Mind for centuries, ever since the Dark One and the Light One were put to sleep. These people think, feel, care, and act. They fall in love, and have children, and face challenges and woes and strife and ultimately death. They are real people, and generations of them have existed within the Dark One's Mind living short and challenging lives where only the smartest and the strongest prevail. They remain trapped in the lair of a god who tests them forever, and as long as the Dark One remains asleep, they will continue to live and die.
However, they know that one day the Dark One will stir, fidget, or even wake up. They dread the day that happens, for with the awakening of the dreamer, the dream will end, and everything the people have fought for will have been in vain.