Shadow is a synthetic hallucinogen drug which is suppose to expand the mind. The drug appears as a green liquid inside a glass vial. It is injected into the neck.
In Episode Nineteen, The Electus corporation injected the Seers with Shadow when they did an experiment on making a human collective consciousness computer. Even after the Seer's freed themselves of Electus, they still continued to drug their own people.
In Episode Twenty, Devon Taltherd admitted his Shadow addiction to Nyx, when she requested some and he had originally refused. He revealed his usage caused the death of a 12 year old girl while he was high during surgery and he envied the Raza crews ability to forget.