A Plagiarus Demon was a tentacled, armored species that could take control of human host bodies. However, they could not be exorcised. It had the power of empathy: if its blood touched a being, it could inculcate this last the memories of this person. A Plagiarus could quickly regenerate; its only weak point was his brain. It was also capable of human speech.
One of these demons victimized a little girl named Hannah. In order temporary seal the demon away, Rupert Giles had to erase Jenny Calendar from his memory, and promised Anne he would later extirpate it. Some time after Giles' death, Angel and Faith faced the same monster. The vampire feigned to drain Hannah in order to draw the monster completely out of her body.