Know that the Warp goes where the Warp wills, into man, beast and machine.
— The Liber Chaotica, Warhammer 40,000
"It's about the worst thing I can imagine... losing who you are like that, all the while being twisted into a damned monster."
Once this must have been a humanoid, but now it is utterly distorted by the forces of chaos. Eyes look at you from many places (there's even one on the left thumb!), showing no signs of a mind, only a rage caused by maddening pain and suffering. As you look, the body twists into other unnatural shapes, growing arms and legs spontaneously, the head twisting and disappearing into the back while a grinning mouth bearing needle-thin teeth opens where proper humanoids have a navel. Only death can stop its suffering, and in a mindless rage it flails madly at everything that comes near it.
— Description of chaos mutant, Ancient Domains of Mystery
The Soviet scientist never discovered what caused the energy spikes in this place, but Halina did. She channelled the anima and then the anti-anima, the Filth - the liquid voice of omnipotent molluscs. Something reached out to the cosmonauts. It cut into them, so that they could cut into the cosmos. There is hunger here that bites holes in space.
There is a pit, full of Filth, held back by a shield of energy. What do you think, dear sweetling, are the chances that itll hold? You will find two doors - a door to Agartha and a door to the Dreaming Prison. Filth constructs detected. Manifested shapes: that of Halinas dead teammates. Assessment: loneliness.
She so longed to see her precious stars. That is how the sleeping whisper tempted Halina. She joined the whisper. The differences between her and the whisper decline. She changes. See the stars in her chest. By and by she shall become the avatar of a malignant fractal. Initiate probability matrix - the only possible outcome is violence.
—Lore Entry on The Facility, The Secret World,
It could hardly even be called an emotion, this thing called Hate. It was more like a demon that took hold of you and consumed you from the inside out, leaving no room in your heart for anything else, until it eventually destroyed you. Your mind was not your own, given over to this demonic presence that mangled and devoured all it touched with as little effort as water freezing.
Hate was impossible to control. The host was not the master. Rather, it was the opposite. There was nothing one could not do while under the influence of this malicious, wicked feeling. There was also nothing one could do of themselves unless Hate allowed them to do so.
"I mean, look at her. Listen to her talk, it's doing something to her mind. She's turning into something, and no one knows what. Our best hope is it's just another monster."
— Caira Diaz on the subject of Kala Kapur, Evolve
Darkness is a pretty tame term to describe what it really is. Its a virus, a sickness. It festers in places you left to abandon and stopped cleaning. Then, when you get exposed to it, it looks for signs of weakness, openings it can use to bypass your defenses. When it does find one, it infects your soul, and starts spreading inside it like a plague would inside your body. It warps you until you are nothing but a grotesque parody of yourself. And then, once its job is done, it tries to get out so it can go do the same to other people. You are not just fighting monsters when you fight the Darkness; you are fighting an epidemic.
"Sometimes... humans have a monster inside them. And it wants us... to catch it."