The Darkness was accidentally released when Stephen threw Evelyn through a decaying wall. The Darkness then assimilated Evelyn into it, and explained through her absorbed soul that the souls that compose it want to assimilate the others in the house into the Darkness as revenge.
Horrified, Steven retreated upstairs, followed closely by the Darkness, which managed to assimilate Watson into it in the process, and then pursued the remaining three. It tried to use Melissa's soul to lure Sara to it to be assimilated, but this failed.The three escaped to the attic, but the Darkness seeped in after them through the floorboard cracks, assimilated Steven, and severed the rope holding the iron gate before Eddie could escape outside with Sara. The Darkness prepared to assimilate Eddie when he revealed that he was adopted. As this meant that he was not involved at all, at this revelation, Watson's ghost allowed Eddie to narrowly escape.
Defeated, the Darkness vanished.
As the Darkness is composed of the non-corporeal ghosts of everyone who died in the Vannacutt asylum, it can seep through the thinnest of gaps like water. The souls that compose the Darkness can also communicate through it.
As most of the ghosts composing the Darkness were patients at the asylum who died either in Vanacutt's experiments, or the fire of 1931, the Darkness will mindlessly attempt to assimilate the descendants of the five surviving staff members, and anyone who gets in the Darkness's way.
However, it appears that descendants who were raised in an orphanage or by a foster-parent will be less bothered by the Darkness once this is revealed, as it somehow means that they were not involved. Also, as all the ghosts are bound to the house by the Baphomet idol, the Darkness cannot leave the house, and thus, if a descendant escapes the house before the Darkness can assimilate them, they will be safe.