Movies and Books
The Philosophers Stone
In the first of the Harry Potters, Hagrid tells Harry about what actually happened to his mother and father in the old house out on the farm. The flashbacks show Voldemort using the famous curse on Lily and James Potter, and attempting to use Avada Kedavra on Harry, but failing. It was then he got the famous scar on his forehead and was known from then on as 'The Boy Who Lived'
Goblet of Fire
Voldemort's return is unknown to the Ministry and to Harry and his friends, but around the wizarding world there have been strange disappearences. These include the loss of an old man by the name of Frank Bryce, a woman who worked for the Ministry, and later on, Cedric Diggory. During the third task of the Triwizard Tournament Harry Potter and Cedric Diggory are racing towards the cup to retrieve the prize. After deciding to grab the trophy at the same time, they fall into a graveyard where Voldemort's mother and father were buried. Wormtail, once James Potter's friend but now loyal servant of the Dark Lord, kills Cedric with the Avada Kedavra curse and Voldemort is returned to a real man form. Harry later escapes with the trophy and is not believed by the Ministry that 'He is Back'.
Order of the Phoenix
The ministry still does not believe Harry, but at the end of Harry Potter and the Order of the Phoenix, Voldemort is confirmed 'returned' and the killing curse, is used. Bellatrix Lestrange is in a deadly and very violent duel with Harry Potter's godfather, Sirius Black. Sirius laughs when Bellatrix misses him the first time, but she succeeds in her next attempt to kill her cousin, using the Avada Kedavra curse and killing Sirius Black.
Just once in Harry Potter and the Half-Blood Prince is the killing curse used. This is performed by Severus Snape, high up in the clock tower, accompanied by Draco Malfoy, Bellatrix Lestrange, and several other Death Eaters. Harry Potter is also at the scene, but hiding underneath watching. Snape steps towards Dumbledore after arriving late, and says slowly, but just as painfully, A-v-a-d-a K-e-d-a-v-r-a!
Avada Kedavra is a curse that can be a range of a few different colors, including blue, green, and somewhere in the middle. Sometimes, it depends on the wand casting the curse as to what color it is. For example, Wormtail and Voldemort produce a green curse, although Snape's is slightly bluer. The killing curse is a form of spell that shoots out of the wand very quickly and gives the opponent not much time to block it. The cast can go off in many directions, shooting up and around the main jet, making it look like a flash of green or blue light.
Mastering Avada Kedavra is a very hard job. The killing curse is one of the hardest of all spells to learn and can only be perfected by very powerful witches and wizards. There is no particular way to say the curse, although you must cast it in a particular way. First, you must hold your wand slightly-not far- into the air. Then you must slowly lower your wand and say the incantation clearly. If you move the wand too fast when casting the Avada Kedavra curse, there could be some major side-effects.