This spell is a corruption of "abracadabra" which was used as a healing spell to drive disease from the patent's body in the middle ages. Its likely source is either the Arabic "abra kadabra" (may the things be destroyed), or the Aramaic "abhadda kedhabhra" (disappear with these words).
Warning! This entry contains spoilers to the Harry Potter Series. Read at your own risk!
There is only one person ever to have survived the Killing Curse: Harry Potter. This was due to the sacrifice of his mother, Lily Evans. The magic was an ancient one, strong enough to cause the spell to rebound on Tom Marvolo Riddle. However, due to Voldemort's Horcruxes, Voldemort survived. The rebound of this curse destroyed the Potter Home in Godric's Hollow, which was unable to be fixed due to the dark magic. The curse also left a mark on Harry Potter's forehead: a thin, lighting-bolt scar that identified him to whomever saw it.
This spell was first seen in Harry Potter and the Philosopher's Stone (movie only) but first named in Harry Potter and the Goblet of Fire, when Mad-Eye Moody demonstrated the Unforgivable Curses to his classes, using this spells on a enlarged spider. It is then used through the next four books.
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