Inferius

"They are corpses, dead bodies that have been bewitched to do a Dark wizard's bidding. Inferi have not been seen for a long time, however, not since Voldemort was last powerful [....] He killed enough people to make an army of them, of course."

- Albus Dumbledore on Inferi

An Inferius (plural: Inferi) is a dead body, reanimated by a Dark Wizard, similar but intrinsically distinct from a zombie. They are created through the magical branch of Dark Arts called Necromancy.

1899
In 1899, it was mentioned that the dark wizard, Gellert Grindelwald, sought to create an army of Inferi, and believed that the Resurrection Stone could grant him such powers. It is unknown as to whether he succeeded, though his conception on the stone's abilities were in fact incorrect, as it brings back souls from the realm of the dead as more-than-ghost but less-than-alive state, not as a living body.

First Wizarding War
During the First Wizarding War, Voldemort was known to have created an army of Inferi from the large amount of people he killed. He placed such an army within the lake of Salazar Slytherin's Locket's hiding place.The Inferi in question were mostly vagrant, homeless Muggles murdered by Voldemort, but some of them were the earthly remains of wizards and witches who "disappeared" without explanation. In 1979, when Regulus Black attempted to destroy the said Horcrux, he succumbed to the Drink of Despair's effects and tried to take a drink of water from the lake, only to trigger the Inferi and was killed by them. Kreacher, his house elf, managed to make it out of the cave with the locket.

Second Wizarding War
The Ministry warned the community that Lord 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.

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 an attempted burglary.

Hunting for Horcruxes
"The wandlight had slid over a fresh patch of water and showed Harry, this time, a dead man lying faceup inches beneath the surface, his open eyes misted as though with cobwebs […]"

- Harry looks at an Inferius in the lake in the Crystal Cave

In 1997, Harry Potter and Albus Dumbledore encountered the very same army of Inferi as Regulus in their attempt to destroy Voldemort's locket Horcrux. At first, they remained inert as the two sailed in the boat towards the central island. However, when Dumbledore suffered from the Drink of Despair's thirst-inducing effects, Harry had to take some water from the lake, triggering the Inferi, who nearly dragged Harry to the depths of the lake to drown. Dumbledore successfully repelled them by conjuring a ring of fire, big enough to surround both Harry and himself. Once all the Inferi returned into the lake, they did not resurface when Dumbledore's ring of fire faltered.

2014 Quidditch World Cup
Malicious and baseless|The ICWQC on rumors that Haiti was using Inferi.|Pottermore}}

In 2014, rumours that the Haiti was using Inferi to intimidate other teams were dismissed by the ICWQC as untrue. However, the rumours proved true when Inferi attacked crowds in the opening ceremony of the 2014 Quidditch World Cup, leading to 300 casualties.

Description
"The Inferius is a corpse that has been reanimated by a Dark wizard's spells. It is not alive, it is merely used like a puppet to do the wizard's bidding."

- Severus Snape regarding Inferi

The Inferi are gaunt, skeletal beings that closely resemble zombies. Being human corpses, they have individual appearances based on the humans they were created from.

Inferi have no free will, and cannot think for themselves; their purpose is merely to serve as puppets of the Dark Wizard who reanimated them. The spells required to reanimate dead bodies are much more complex than those used to make inanimate objects fly. As no spell can bring back the dead, the Inferi are not alive, just corpse puppets. Despite this, they can apparently speak.

Due to their status as being unfeeling dead, the Inferi are immune to bodily damages such as slashing, and have great physical strength, enough to kill a human or drag them away. Due to their superior strength, they are especially dangerous en masse.

Defence against Inferi
"However, like many creatures that dwell in cold and darkness, they fear light and warmth, [...] Fire, Harry."

- Albus Dumbledore on the defence against Inferius

Because Inferi are creatures of the dark, they dislike light and heat (no spell has been found to render dead flesh impervious to burning. The most effective spell against them is therefore a fire-summoning spell, such as Firestorm. Because of this, the Inferi are enchanted by their master to avoid flames. 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 such as the Cruciatus Curse. This is evident when Harry Potter attempts to use the dark spell Sectumsempra on the Inferi in the Crystal Cave, as they have no blood to spill.

Etymology
Inferius is a tampered form of the Latin word for "underneath; below", Inferus, but with an obvious connotation of being 'lesser' than a living human. Meanwhile, Inferi means "the dead; underworld". J. K. Rowling has changed this real Latin words to form these creatures.

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 Hangleton Graveyard as part of an "army of creatures whom all fear".
 * Inferi may be incredibly strong as a picture in Snape's Defence Against the Dark Arts class shows a bloody mass on the floor, attributing to a person provoking the anger of an Inferius. Alternatively, this could be attributed to their viciousness and implacability.
 * Zombies are different from Inferi, although they have much in common. It seems that zombies are naturally formed, while Inferi are artificially created. J. K. Rowling decided against naming the Inferi zombies because zombies are not part of British folklore. Also, one tradition has the sorcerer using his soul, or part of it, to sustain himself, which she decided conflicted with her Horcrux story.
 * In first part video game adaptation of Harry Potter and the Deathly Hallows, Inferi appear again, despite them not being in the mainstream storyline.

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 1 (video game)
 * The Tales of Beedle the Bard (real)
 * Harry Potter: A Pop-Up Book
 * LEGO Harry Potter: Years 5-7
 * Pottermore