The relentless assassins known collectively as The Chain are widely believed to be members of an ancient death-cult, they are not.
In point of fact, the only "members" of The Chain are a pair of small humunculi who use humans as hosts while carrying out their assassination contracts and speaking to clients. The pair have lived for centuries, moving from body to body, growing rich.
Although the humunculi are each about the size and shape of a newborn human, they are able to squeeze themselves into the head of a human host through the mouth (usually while the host-to-be is asleep or drunk). Once inside, the humunculi eat the host's mind and take its place in the cranial cavity--this allows the humunculus to control all of the hosts' actions. A possessed gestalt assassin is stronger, faster, and more resilient than the original host body, and has hundreds of years of training in both armed and unarmed combat.
Once they accept a contract, the humunculi of The Chain will not stop, and will attack from any available body until their quarry is slain--often pursuing a target for decades. If a host is slain, the humunculus will generally exit through the eyesocket (leaving the empty skull to collapse) and crawl quickly off to occupy a new host.