Mendelov's Marvelous Rainbow Tavern Company was a front given to the name of a band of Dragonborn, pretending to be a mere tavern company. In truth, they had the aim of completing a ritual to bring Tiamat into the world.
The Company was brought in by Mayor Ballard to help New Newfoundland Land return to prosperity after an economic downturn. They begin systematically killing off people in town to use in their ritual. The Baby-sitters Club caught wind of this and attempted to stop them, but were unsuccessful.
Following the completion of the ritual, and the rise of Tiamat, it is unclear is the Company is still alive, or if they were also all killed. Time will tell.