An Inferius (Plural: Inferi) is a dead body wich is reanimated by a dark wizard in the Harry Potter franchise. An Inferius can be compared to a zombie. They serve the Dark Wizard as a slave. They were used by Voldemort during the first and the second wizarding war. An Inferius is a dark creature and this means that he dislikes light or fire. Most of the fire-spells work against Inferi.
Encounters with the Inferis
In 1979, Sirius Black's brother, Regulus Black tried to steal Slytherin's locket. (One of Voldemort's Horcruxes) He was killed by the Inferi's and told his house-Elf Kreacher to bring the real locket to the Black Mansion, which Kreacher did.
In 1997, Harry Potter and his headmaster Albus Dumbledore went to the cave on the search for Voldemort's Horcruxes. Harry was pulled underwater by the Inferi's. But luckily he was saved by Dumbledore who used a spell that casted a lot of fire wich chased away the Inferi's.