Into the Gloaming
Sean Farrell smuggled large amounts of opium into Arkham hidden away in jars of orange marmalade. For lack of a more suitable term, the drug itself has taken that eponym.
The opium contains an unknown contaminate — flecks of a brownish material.
When injected into a rat as an experiment, the rat exhibited symptoms typical of opiate use: lethargy, elation, and eventual withdrawals. Two days after the injection, the rat died. A few hours later, it came back to life.