An Inferius (plural: Inferi) is a corpse that has been reanimated by a Dark wizard’s curse. It becomes a grisly puppet, and may be used as an expendable servant by the Dark wizard in question. The most obvious sign that one is facing an Inferius rather than a living human are the white and cloudy eyes.
The spells used to reanimate a human body are much more complex than those used, for instance, to make inanimate objects fly. The Inferius may be cursed to respond lethally if disturbed, to kill indiscriminately, and to undertake perilous jobs for its master. Its limitations are, however, obvious; it has no will and no brain of its own, and will not be able to think its way out of unforeseen trouble. As a warrior or guardian with no regard for its own safety, however, it has many uses.
The Inferi whom Harry and Dumbledore encounter in the depths of the lake in Harry Potter and the Half-Blood Prince were, when alive, mostly vagrant, homeless Muggles whom Voldemort had murdered for the purpose during his first rise to power, although some were the earthly remains of wizards or witches who ‘disappeared’ without explanation.
Preserved indefinitely by Dark magic, an Inferius can only be destroyed by fire, for no spell has been found to render dead flesh impervious to burning. Inferi are therefore enchanted to avoid flames by their master.
Inferi have much in common with zombies, which are mentioned as separate creatures within Harry’s world. I had several good reasons for not wishing to call the guardians of the locket Horcrux ‘zombies’. Firstly, zombies are not part of British folklore, but associated with the myths of Haiti and parts of Africa. While the students of Hogwarts would learn about them, they would not expect to meet them walking down the streets of Hogsmeade. Secondly, while zombies of the Vodou tradition can be nothing more than reanimated corpses, a separate but related tradition has it that the sorcerer uses their souls, or part of their souls, to sustain himself. This conflicted with my Horcrux story, and I did not wish to suggest that Voldemort had any more use for his Inferi than as guards of his Horcrux. Lastly, zombies have been represented and reinterpreted on film so often in the last fifty years that they have a whole raft of associations that were of no use to me. I’m part of the Thriller generation; to me, a zombie will always mean Michael Jackson in a bright red bomber jacket.
The name Inferius was a play on ‘Inferus’, which is Latin for ‘below’, but with an obvious connotation of being ‘lesser’ than a living human. ‘Inferi’ means the underworld.