what is the black mass
The black mass is the meeting the devil is holding in the woods. It is where Goody Cloyse, Deacon Gookin, and the minister all heading. It is where Brown is heading too, though he doesn't realize it until he already in the midst of it. We get some foreshadowing about what is going to happen there. From Goody Cloyse we learn that " . . . for they tell me there is a nice young man to be taken into communion to-night." From Deacon Gookin and the minister we learn that "Moreover, there is a goodly young woman to be taken into communion." The reader can infer that they are referring to Brown and Faith.
The black mass is also where the devil gives his best speech, welcoming the devil worshippers to "the communion of your race." By this the devil is illustrating for the Salem villagers, and humans in general, for there are devil worshippers from the surrounding towns there as well as Native Americans, how evil is the nature of humanity, not good. He goes on to explain how only here at the black mass, where they can look upon each other without guilt or shame because they all have sin in common, are they united. In Salem village, because of the Puritan religion, people are separated into heathens, sinners, and the 'elect' (those who are destined for heaven).