If targeted at a group of individuals and with the intention to kill them, could a single invocation of Avada Kedavra eliminate an entire group? I am asking for, er, a friend who has, um, a pest problem.
For example, if I aim over an anthill and I mean to kill all the ants, will I have to kill one at a time? Again, asking for a friend. I don't have any problem with the ants.
It seems like once Avada Kedavra hits one living target, it’s “used up”. Fawkes was able to take a Killing Curse for Dumbledore. Once it hit him, it couldn’t hit Dumbledore.
“But even as he shouted, another jet of green light flew at Dumbledore from Voldemort’s wand and the snake struck –
Fawkes swooped down in front of Dumbledore, opened his beak wide and swallowed the jet of green light whole: he burst into flame and fell to the floor, small, wrinkled and flightless.”
- Harry Potter and the Order of the Phoenix, Chapter 36 (The Only One He Ever Feared)
For blocking the Killing Curse like this to be effective, Fawkes would have needed to be able to take the whole of it, with none of it being left over to still hit Dumbledore. Fawkes is also fairly small, so if he could block a whole Killing Curse, that shows it’s likely to be a one-target-only spell.
If there were two souls sharing space in the same body, like Quirrell being possessed by the Dark Lord, or Harry and the piece of soul in him, it’s possible that a single Killing Curse could kill both souls that were housed in the same body. One single Killing Curse killed the piece of the Dark Lord’s soul in Harry and would have killed Harry as well if it wasn’t for his mother’s sacrifice in the Dark Lord’s blood (and to a lesser degree his being the master of the Elder Wand).
Bellatrix has found a good example from the books of Avada Kedavra seemingly only affecting one target at a time. I think that her conclusion is probably right, although the case of Wormtail raises a question mark.
We know that Wormtail (framing Sirius Black) killed thirteen people with a single spell.
...the magical community lives in fear of a massacre like that of twelve years ago, when Black murdered thirteen people with a single curse.
[...]
"Anyway, they cornered Black in the middle of a street full of Muggles an' Black took out 'is wand and 'e blasted 'alf the street apart, an' a wizard got it, an' so did a dozen Muggles what got in the way."
(Prisoner of Azkaban, Chapter 3, The Knight Bus).
It's possible that Wormtail used Avada Kedavra but we're never actually told what spell he employed. It's been speculated that Wormtail used Confringo or some such spell to blow up an actual gas line since a gas explosion was the cover story the Ministry came up with to explain the atrocity to the Muggles. Backing up this suggestion is Black's testimony that Wormtail "blew apart" the street.
"Then, before I could curse him, he blew apart the street with the wand behind his back, killed everyone within twenty feet of himself - and sped down into the sewer with the other rats..."
(Prisoner of Azkaban, Chapter 19, The Servant of Lord Voldemort).
Cornelius Fudge also describes a big crater in the street.
"I was a Junior Minister in the Department of Magical Catastrophes at the time, and I was one of the first on the scene after Black murdered all those people. I - I will never forget it. I still dream about it sometimes. A crater in the middle of the street, so deep it had cracked the sewer below. Bodies everywhere. Muggles screaming."
(Prisoner of Azkaban, Chapter 10, The Marauder's Map).
There isn't a definitive confirmation anywhere in canon, however. What spell Wormtail used remains a mystery.
If he did use AK then it would be the only example we have of someone using AK on multiple targets. This in itself may be an argument against Wormtail having used it - if you can use AK to kill groups of people then why didn't we see it being used that way in the Wizarding Wars?
Furthermore, Wormtail's crime is presented as extreme, and probably unique, in its scale and brutality.
"He murdered thirteen people?" said Harry, handing the page back to Stan, "with one curse?"
(Prisoner of Azkaban, Chapter 3, The Knight Bus).
AK is, well, not commonplace exactly, but certainly not unheard of in a wartime context. We see it being used frequently in the final three books. Wormtail's spell seems to be in a separate category of destructiveness, suggesting that Wormtail did not use AK.
We also don't see any reference in any of the descriptions of the attack of the distinctive green glow that accompanies Avada Kedavra. This again suggests that Wormtail didn't use it.
A counterargument in favour of Wormtail using AK is that the spell seems to be more destructive when the person casting it does so with great power and intent. In other words, there are different grades of force when someone casts AK.
"Avada Kedavra's a curse that needs a powerful bit of magic behind it - you could all your wands out now and point them at me and say the words, and I doubt that I'd get so much as a nose-bleed."
(Goblet of Fire, Chapter 14, The Unforgivable Curses).
We also know that with the Unforgivable Curses that you need to mean them when you cast them. If Wormtail was harbouring a lot of internal pain and hatred then he may have been capable of casting a very powerful AK (although he's never presented as being a particularly talented wizard).
All in all, it doesn't seem likely that Wormtail used Avada Kedavra to kill his targets, although if he did then this would be the sole example of Avada Kedavra being used to kill multiple people.
Even if the spell does work on multiple targets it wouldn't be much use for your ants, unless you're prepared to kill everyone in a twenty foot radias. I'd recommend Bombarda or a good old-fashioned foot.
Unknown
There is no instance in either books or movies where the Killing Curse was cast at more than one individual at a time. That, and the one-to-one nature of hexes, jinxes, and curses in the HP universe tend to make me think that the answer is likely "no".
That said, the true answer will have to come from JKR herself. You could easily ask on her site.
Bellatrix's answer is superior to mine, which see.
Avada Kedavra is a spell that has spatial properties.That is to say that the spell does not simply kill an intended victim, but instead it has to be aimed at a specific target and it has to hit the target. Consider, for example, the duel between Dumbledore and Voldemort in Chapter Thirty-Six of Order of the Phoenix in which Voldemort sent a Killing Curse at Dumbledore but it missed:
He sent another Killing Curse at Dumbledore but missed, instead hitting the security guards desk, which burst into flame.
The spatialness of Avada Kedavra is represented as a jet of green light, as in Harry's duel with Voldemort in Chapter Thirty-Four of Goblet of Fire:
A jet of green light issued from Voldemort's wand just as a jet of red light blasted from Harry's — they met in midair — and suddenly Harry's wand was vibrating as though an electric charge were surging through it; his hand seized up around it; he couldn't have released it if he'd wanted to — and a narrow beam of light connected the two wands, neither red nor green, but bright, deep gold.
It stands to reason, then, that something which gets hit by the jet of light gets killed, and something which does not get hit by the jet of light does not get killed. Therefore, if multiple creatures could be hit by the same jet of light, then perhaps multiple creatures can be killed with just the one spell. In the example here of ants, if the ants are small enough that thickness of the beam of light can cover all of them, then why shouldn't all of them die?
