Heaven's Slave was the name given to the hyper-addictive drug that Shijima, a student who attended college with Nakura and Izaya, created, as well as the alias used by the Experimental Legal Drug Club after it initiated its distribution activities. It is implied that this group resulted from the one that Izaya began in college along with Nakura and then Shijima.
Heaven Slave Tattoo

Heaven's Slave symbol as shown in Durarara!! x2, Episode 15 on a young man's neck as a temporary tattoo

This Experimental Legal Drug club, when left to Shijima, allowed him to eventually produce Heaven's Slave and use Nakura - through his alias, "Kumoi" - as a scapegoat. It was said by Mikiya Awakusu that Heaven's Slave was inspired by the Dollars to take their drug-dealing business online and connect with potential buyers there. 

When a small group of Heaven's Slave college students heard that there was a reward for killing Akabayashi, they went on the Dollars site to locate him. Unknowingly following information fed to them by the yakuza executive himself, the group ended up facing off against his enemies to destroy one another while he casually walked away from the scene. Due to Akabayashi's history and reasoning regarding his intense hate for drugs and drug-dealers as well as their encroachment on the Awakusu group's territory.

The distribution gang later became rivals with Amphisbaena in an attempt to establish dominance as the second strongest gang in Ikebukuro. Ultimately the production of the drug is halted after Izaya Orihara impersonating Nakura to destroy both organizations likely for bringing attention to Nakura that wasn't carefully controlled by Izaya.

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