Transmission & Vectors
Through a blood ritual, a piece of the Dark parasitically attaches to the host's body, whispering into his ear. It usually takes on the form of an ink blot somewhere on the host's body or an unusually solid shadow.
Greed and dark magic.
Looking like you're talking to yourself, nightmares, hallucinations, sweating, greater than average strength and intelligence. As the darkness spreads: paranoia, intrusive thoughts, inability to sleep, veins showing through skin, obsessive behavior, schizophrenia, bodily deformities even as abilities increase.
The Darkness can never be purged.
Dark power inevitably leads to insanity and undeath as a shadow creature.
The children of those affected by the Dark will be deformed and otherwise lacking, but will also be stronger and have innate Dark powers; basically all of the effects without an active corrupting influence.
Greedy or egotistical people, politicians, anyone who wants power and will do anything to get it. Children of a host are often born with physical or mental abnormalities. (See Magnus).
Hosts & Carriers
DON'T TALK TO TSICAR. DON'T LISTEN TO TSICAR. DON'T LISTEN TO PEOPLE WHO TALK TO TSICAR. THIS HAS BEEN A PSA.
The host can convince others to join him. This is usually unsuccessful-- no one listens to a creep with a shadow you can literally trip on.
Tsicar, evil king, knew of the Darkness from his time as a war god. He harnessed its corrupting influence as a way to gain an advantage against Oreh in the Shadow War. Being a god, he was able to master this force and use it on others.
Hosts are described as crazy and cast out. This is for the better.