The Macri is an extremely rare species of abnormal spider that has a psychic connections to a larger genetic cousin called Big Bertha.
Macri need a warm blooded host in order to survive and once they find a host the two develop a symbiotic relationship which means a macri host will die if the macri leaves their body.
Macri host develop psychic connection with Big Bertha and can even command her to obey their will.
Even after the macri left Will's body he still retained his psychic connection with Big Bertha and could still see her avatar.
A macri can be forcibly removed from its' current host if that host is hit with a low frequency magnetic pulse which will drive the macri out. However, the Cult of Kali believes that the macri must be free to choose its' host or else the host won't be able to see Big Bertha's avatar.
The Cult of Kali's human members allowed them to crawl into their mouths and give them visions of future disasters; as a side effect, the host's life is prolonged as long as the macri is inside him. When the Marci leaves the body the host dies.
"Macri" or "Makri" is actually an Urdu and Hindi word for spider.