Little is known about Blackheart Spirits. Althought according to the events in Kingdom of Liars Episode 3 they seem to be very evil and are willing to comunicate with cultists.
Kingdom of Liars 3Edit
When the Unnamed Protagonist looks for clues in a deserted house, he stumbles across an open book with a dagger drenched in blood next to it. The open pages of the book state that the book belongs to a cultist that is in service of a Blackheart spirit. And it becomes clear the events in Shadetale all went according to the plan of this evil spirit. The spirit states that he was willing to sacrifice the Archdemon in order to free the thousand Fiends.