Inferius

An Inferius (plural: Inferi) is the dead body of a human reanimated by a Dark Wizard, similar to a zombie. They have no free will, and cannot think for themselves; their purpose is merely to serve as puppets of the Dark Wizard who reanimated them. They were used by Voldemort in both the First Wizarding War and the Second Wizarding War. It is mentioned that the Dark Wizard Gellert Grindelwald also sought to create an army of Inferi. It is unknown as to whether he succeeded. Inferi are created through the magical branch of Dark Arts called Necromancy.

Defense against Inferi
Because Inferi are creatures of the dark, they dislike light and heat. The most effective spell against them is therefore a fire-summoning spell, such as Incendio, Lacarnum Inflammare, or Fiendfyre. Other spells might work against a few Inferi, but might not be useful against a whole army of them. It must be noted that Inferi are resistant to most spells: they cannot feel pain, and thus are unaffected by many attacks whose main effect is pain. This is evident when Harry Potter attempts to use the dark spell Sectumsempra on the Inferi in the Horcrux cave.

Encounters

 * In 1979, Regulus Black betrayed Voldemort and attempted to steal Salazar Slytherin's Locket. He was killed by an army of Inferi in his attempt and his house-elf Kreacher took the locket and hid it in the Blacks' mansion.
 * In 1997, Harry Potter and Albus Dumbledore encountered an army of Inferi in their attempt to steal one of Voldemort's Horcruxes. Dumbledore successfully repelled them by conjuring a ring of fire big enough to surround both Harry and himself.
 * Not exactly counted as a real encounter, but worth mentioning: Mundungus Fletcher was arrested and put into Azkaban because he disguised himself as an Inferius after being caught during a burglary.
 * The Ministry warned the community that Voldemort may be using Inferi once again in the Second Wizarding War, and he was mentioned that he did use as much as an army during the First War.

Etymology
"Inferius" is Latin for "underneath; below." It could also be a tampered form of "inferus", or "the dead; underworld." As J. K. Rowling sometimes changes real Latin words to form some incantations, or names of creatures or people, it could well be that the second meaning is the correct one.

Behind the scenes

 * According to W.O.M.B.A.T., Inferi may have the ability to speak.
 * Inferi may have been briefly referred to by Voldemort in the Little Haggleton Graveyard as part of an "army of creatures whom all fear".
 * Inferi may be incredibly strong as a picture in Snapes' DADA class shows a bloody mess on the floor, attributting to a person provoking the anger of an Inferius.

Appearances

 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Half-Blood Prince (video game)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part I (video game)
 * The Tales of Beedle the Bard

Notes and references
Inferi Инфернал Manalius