The Chain were a previously unknown power who turned up in the Alpha Quadrant in 2385.
The Chain were made up of a previously unknown species, one said to have grey colored skin, with many lines and angles on their face, along with dark markings around the throat. These markings changed to pitch black whenever any member of the species experienced anger.
Sometime before 2385, the Chain had been observing the events of the Alpha Quadrant, indicating a rudimentary knowledge of the affairs of the Quadrant. The Chain were technologically advanced, possessing technology that was a bit ahead of the major powers of the Alpha Quadrant. A noted example of this advance came in the form of transwarp beaming, which was working well, but still relatively unsafe for frequent use. The Chain ship that was encountered had been heavily armed and dwarfed an Olympic-class Starfleet vessel.
Although the Chain were technologically advanced, an offshoot of the Chain, the People of the Open Sky, indicated the state of the Chain homeworld was rife with poverty and inequality. The poor were left to fend for themselves, while the rich lived in luxury and only those in the highest castes of society were accepted to be leaders, even when it came to the military.
In the eyes of the Chain, the People of the Open Sky were nothing more than separatists, who had a tendency to abduct children and mistreat them. The arrival of the People of the Open Sky in the Alpha Quadrant attracted the presence of the Chain, who attempted to track them.
In November of 2385, the Chain sought the location of the People of the Open Sky, who had been taking refuge at Federation space station Deep Space 9. Though unsure where they were, they stopped the first ship they encountered, the Athene Donald. Upon meeting the crew, they let themselves receive a standard tour of the vessel, from which they showed an obvious amount of boredom. It wasn't until one of the vessel's crew let it slip that the People of the Open Sky were on Deep Space 9, unaware they had been after them, that they were shaken into action. The Chain vessel demanded the handing over of the People of the Open Sky, or else face destruction. The Chain were only prompted to stand down upon talking to the de facto leader of the People of the Open Sky, Oioli, an old friend of the captain of the Chain vessel. The two came to an agreement and the People were permitted to go about their business, with the Chain vessel turning around for home. (DS9 novel: The Missing)