The Dark Hearts are a cabal of thirteen magic users, a varied collection of wizards, witches, enchanters, sorcerers, warlocks etc.
They created a number of heavily reinforced, enchanted stone golems to use as proxy bodies, and a pocket dimension to preserve their minds which serves as a network connecting to their golems as a means of obtaining immortality. Any of the Dark Hearts can assume control of one of their golems from anywhere, even between different planes, since they have a few in the Demon Realms as well.
They came into conflict with Containment, who managed to destroy one of their facilities and collect one of their golems. Rather than fight back, the Dark Hearts arranged to provide funding for Containment, installed one of their associates in a managerial position, and steered the group away from their work by pointing them at their more disruptive rivals.
When the portal between the demon realms and Earth collapsed, they began working directly with the Courier's Guild to try to fix it. As part of that, one of them was enlisted to teach Erin how to use magic so that she could participate in the reconstruction of the portal.
When dormant their golems do not retain humanoid form, instead resembling some form of crystalline formation. When a Dark Heart takes control of one, their personal mask will materialize above it, causing the golem to assume its active form. A golem's form can be personalized, but is often restricted to what the intended purpose of the golem is; a simple communication model or attack model cannot preform complex magic, as they lack the dexterity for it. They are known to have at least one golem specifically made to enable the Dark Hearts to teach others.
It's been noted that volume control can be difficult when your mind is separated from your body, so some of the Dark Hearts have a tendency to shout when speaking through a golem.