The Shedim are bodysnatching free spirits from a far metaplane, capable of manifesting in the physical world only by anchoring themselves to dead bodies (or 'unoccupied' ones, in the case of those who are using astral projection.) They seem to have an inborn hatred for all living things, and this hatred drives them to attempt to destroy metahumans and spirits alike.
After their first reported appearances in 2061 (The Year of the Comet), the Shedim are known to have influenced events in the Middle East, specifically relating to the rise of the New Islamic Jihad. In 2064, this plot is foiled with the revelation that the leader of the Jihad, Ibn Eisa, had been possessed by a Shedim.
Observation of the shedim has revealed that the amount of intelligence, motivation, and malice among them varies greatly, leading some to theorize that like insect spirits, the shedim are separated into multiple categories or subspecies. The rank and file normal shedim take the role of servants and drones, while the master shedim are the puppet-masters, guiding their lesser brethren in various plots. The master shedim are more intelligent, more powerful, and have the ability to open astral gateways, which they used to bring forth more shedim to suit their purposes.
On the astral plane, shedim appear as translucent apparitions that resemble large jellyfish. Due to their apparent interest in causing death, chaos, genocide and war wherever possible, some have theorized that the shedim may be somehow linked to the Wraith and be an early type of a Horrors, heralding the coming of a Scourge.