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; their purpose is merely to serve as puppets of the Dark Wizard. They were used by Voldemort in both the First Wizarding War and the Second Wizarding War. Inferi are created through the magical branch of Dark Art, Necromancy.

Defence 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, considering they cannot feel pain and thus resist many attacks whose main effect is pain.

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.

Known Victims

 * Regulus Black

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.

Appearances

 * Harry Potter and the Goblet of Fire (video game)
 * 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
 * The Tales of Beedle the Bard

Notes and references
Inferi Инфернал