Soy Sauce is the slang term of a new-to-market drug in the John Dies universe that is a driving force of the series' plot and a founding point of the first books' lore. Also called "The Sauce" or "Flying Black Shit", the drug is lethal to all who ingest it, and it is implied to be sentient or self-aware in some way. Those who consume the sauce die in gruesome ways, such as spontaneous combustion, dehydration, or suicide; while others simply cease to exist. To date, the only known survivors of the drug are David Wong, John Cheese, and Molly, all of whom suffer severely as a consequence of the sauce.
The books official summary describes the sauce as "a drug that causes its users to drift between dimensions, [to a place where] not all can return from".
The sauce is described as being almost watery in viscosity, as well as being 'darker than the darkest black'. It is capable of being frozen, microwaved, burned, injected, or smoked. When inspected through a microscope, the sauce appears to be squirming or covered in spikes. It bubbles occasionally.
Soy Sauce can infect others through blood contact with a user by a non-user, and is not available on the traditional drug market.
The Soy Sauce is a sentient fluid that is capable of going to almost any length to thrust itself upon unwilling users. It wants to be ingested and forces its victims to consume it in horrendous ways if they refuse to directly inject it into their bodies. By default, the Sauce is a liquid; however, it can take the form of solid capsules, of dissoluble tablets, or of living creatures and can also manipulate its own movements to force its way into a victim's body in ways such as sprouting wings and dive-bombing them, or by attempting to jump off the tip of a needle.
The Sauce's purpose is to turn the user into a portal from one dimension to another, which allows other-worldly creatures to pass through them. When on the Sauce, a victim can slip between dimensions, manipulate their own appearance, float or hover, alter the events of their reality, see the future, travel through time, read minds, freeze space, disconnect their mind from their body, and see things that would otherwise be invisible to the human eye. In extreme cases, Soy Sauce is even hinted to have resuscitative properties.
While the superpower-esque side effects only last about eight hours, the user will remain a "door" to alternate dimensions for the rest of their life. Because of this, most who experience the drug die after they have served or failed to serve their purpose. However, some side effects may linger in those who survive their initial encounter with the sauce- for example, John and Dave can see invisible things, even without taking the Soy Sauce- although the long term reach of its effects are suspected by Dave to "fuck [you] up for life", or possibly longer.
John and Dave continue to use the Soy Sauce while investigating the paranormal in order to fully perceive the phenomena of Undisclosed.
It is not known exactly where the Soy Sauce comes from, but it is implied to be from the same dimension as Korrok. John bought it from Robert Marley at a party, and used it to 'freak out' his friends later. He and Dave eventually discover an alien species (nicknamed the "Wig Monsters") that secrete Soy Sauce out of their scorpion-like stingers when frightened; but it is not certain that they are the original source of all Soy Sauce.
Later in the first book, a pillar made entirely of Soy Sauce appears in a cave deep under the abandoned mall; this is possibly where Robert first contracted it.