"A dark coven has formed in Tristram and I fear that their superstitious violence may damn the whole region. I discovered very little during my eavesdropping--except that the word "Maghda" was repeated several times in their chants. Oh well, now I journey to the safety of my home, where I may put these worries behind me."
Dark Cultists are members of the Dark Coven.
Dark Cultists are human mages who spawn with Berserkers. Cultists are very weak, but they can shoot fireball projectiles and summon powerful Dark Demons, so it's wise to bypass the hulking Berserkers to kill the Cultists as quickly as possible.
Cultists can be recognized from Dark Vessels by their lack of a staff. Vessels have a long staff which curls at the end, on which they lean while summoning up the demons by which they are possessed.