The Still And Lightless Beast is an entity that stalked the Montauk family from at least the year 1990 until 2002.
It was very likely the cause of both Linette and Robert Montauk's deaths, seemingly killing them both for defecting from and betraying the interests of The People's Church of the Divine Host.
The nature of the entity is difficult to ascertain, as it is accompanied by a dimming or total extinguishing of light upon its arrival. Although Julia Montauk described it as sounding like wet meat as it knocked on her door, and strong enough to break the door down, and that it made a growling noise.
The Beast was sacrificed by The People's Church in an attempt at a ritual in March 2015, meaning to trigger an apocalyptic event that would bring The Dark into the world.