Also known as Slow Death, Death, or just plain D, Substance D is a powerful psychoactive drug derived from a small blue-flowering plant, Mors ontologica.
The drug dampens the links between the left and right hemispheres of the brain, causing the two hemispheres to function independently. As a side effect, the user experiences vivid hallucinations (which are a side effect of the two sundered hemispheres attempting to reconnect). As the nickname Slow Death suggests, it is lethally addictive; long-term use can lead to two separate, mutually unaware personalities.
Substance D is produced at farms run by an organisation called New Path. Here, the Mors ontologica flowers are grown, and refined into the drug. These farms are staffed primarily by addicts of the drug, sent to them for their rehabilitation.