Two long-standing conundrums in the fandom.

1. Does the Killing Curse work on magical beasts like dragons, manticores, and nundus? It is described as unblockable which would suggest it would work. But if so, why are these beasts considered so dangerous even to large groups of wizards? A single wizard capable of the Killing Curse should be sufficient to handle them.

2. In duels involving powerful dark wizards, why do they use any directly offensive spell other than the Killing Curse?

I would like to suggest a canon-compatible mechanic for the Killing Curse which addresses both of these points. It's all based on the fake Moody quote in GoF:

There's a couple of points to draw out from this quote:

1. It's possible to cast the Killing Curse with varying levels of power.

2. The Killing Curse only actually kills if it meets a certain threshold of being sufficiently powerful. If it's beneath that threshold it simply causes minor injury.

So the supposed "unblockability" of the Killing Curse comes with a major caveat: it's only true in the case of Killing Curses of sufficient power to gain that effect. We might call that level of power the "killing threshold".

This immediately gives you a partial answer to the second conundrum: most wizards are simply insufficiently powerful to meet the killing threshold. Those wizards are better off using other directly offensive spells. And consistently with this, when we do see Voldemort duel (who is certainly above the threshold), he uses the Killing Curse a lot. It's his main offensive spell, with his other magic largely being used (a) defensively and (b) to manoeuvre the opponent into a position where they can be hit with a Killing Curse.

But this still doesn't explain the magical creature conundrum, nor why we see, for example, Bellatrix using directly offensive spells other than the Killing Curse against Molly.

So far we've stuck pretty close to canon. This next step, however, requires a speculative step. That speculation is canon-compatible but it is still pure speculation.

Here's the speculative addition to the mechanic:

3. Different beings have different killing thresholds, in proportion to their magical power.

Basically, the more magical a being, the higher the killing threshold. Dragons and nundus, as highly magical beings, have a killing threshold so high that very few wizards, if any, would be able to muster the power required. But if a wizard is able to meet that threshold, then it remains true that the Killing Curse is unblockable and will instantly kill that creature.

That explains the creature side of things quite satisfactorily, I think. But there remains one final speculative addition to make to the mechanic, which is a development of the third. You've probably guessed it:

4. Different wizards have different killing thresholds, in proportion to their magical power.

Just like creatures, to kill a wizard with the Killing Curse, your curse must be sufficiently powerful to meet that wizard's killing threshold. That threshold will change depending on who you are cursing. Killing a powerful wizard like Voldemort requires a substantially more powerful Killing Curse than killing a weak wizard like Crabbe.

Essentially, if you use a Killing Curse against another wizard, you have entered into a dick measuring contest where your magical power is directly in comparison to your target's. Only if your curse is of sufficient power will it succeed in killing them. You had therefore better be certain that you're more powerful than your target if you're going to use the Killing Curse on them - which again explains why Voldemort is happy to throw them around like candy.

 

What if the reason they wouldn't be able to kill him was because they were only casting it ineptly? I'm pretty sure that in Transfiguration, they tried some spells that they didn't quite get on the first try, like the first spell was turning a match into a needle, and some of them only managed to make it silver. You need to understand and practice spells before casting them. With dark magic, however, there's another requirement.

I would say that the reason Voldemort tosses them around like candy is because he is an evil man and can easily manufacture hatred for anyone in the world- small children, allies who displease him, and old muggles who were just in the way. I would venture that most witches and wizards not only refrain from practicing killing curses because they're illegal to use under any circumstances, but because practicing properly most likely requires killing several people to get it to where you can reliably kill any target.

 

Interesting stuff!

About dragons, and possibly other magical creatures as well, could it be that the scales on their body deflect all curses, including the killing curse? We find out that a dragon's weak point is its eyes, and a killing curse straight in the eye might work, but it would be difficult to get the aim right. When Harry attempts to stun the skrewt in GoF in the labyrinth, they kept bouncing off because it hit the shell of the animal, but the spells that hit the skrewt's belly worked. I realize it might be that the killing curse is an exception to this, but still a thought.

 

Nothing in the book lead to believe the killing curse require to be powerful. I always understood it as requiring a clear and strong intent to kill the other person.

For example, Harry failed to cast the cruciatus curse because he did not really want to her in pain, he was just angry.

That is why they sent you directly to prison for using them: the mens rea is proved by the actus reus - if a wizard was able to cast the spell, he had a guilty conscience. Nothing to do with power.

As for unblockeable, it only work against magical shield as we see in Book 5 when Dumbledore is able to use physical or living obstacles to block the killing curse. It can also be dodged.

Magical beasts scales/skin might be resilient enough to block the killing curse and they would only be injured by it.

In conclusion, if the killing curse was a power measuring contest, Harry would not be the only one to have survived the killing curse in history.

 

You mean nothing except the part I quoted which states exactly that, which is the only time we're ever told anything about how the Killing Curse works?

Moody/Barty states in the classroom that they could cast Killing Curses at him and he would survive because they wouldn't have enough power behind them. So people surviving weak Killing Curses is not considered unusual or special. What is special about Harry is that he the only person to have survived a Killing Curse which should have worked.

 

You are right, I read too fast.
We do not know what powerful mean in that contest. He could have meant no one wanted him dead enough to have an impact (intent) or that magical power level were involved.

If it is power level, how is it defined? Could Malfoy have killed Dumbledore with the killing curse, Pettigrew, etc.

How do we know Voldemort spell should have worked? For all we know, he could have made a casting mistake (unlikely but it could happen). As a reader, we know there was a lot involved: Snape requests for Lily to be spared, Voldemort agreement (if memory serves), Lily sacrifice, etc.

If surviving the killing curse (botched) was no big deal, why did people jump to the conclusion it should have worked?

We do not have enough to reach a definitive answer unless Rowlings clarify the situation.

 

On the one hand, this sounds fair, but no one ever makes this distinction. It's always that he's the only person to survive the Killing Curse, period.

 

One interesting thing is how the Killing Curse affected the Statues animated by Dumbledore in the Order of the Phoenix at the Ministry.

The First thing we see is one of them just tanking a Killing Curse that was meant for Harry.

The next time we see one of them Bellatrix uses several spells on it, but is unable to harm it, the Text is not clear if she uses the Killing Curse.

Later we see one take a hit intended for Dumbledore, but this time its destroyed.

Going with your idea of 'killing thresholds' you would have to assume that your intended target also factors in on the amount of power you put into the curse, since we see that a Killing Curse intended for Harry isn't enough to destroy the statue, but one intended for Dumbledore is.

Overall I think this support the Idea though that not every Killing Curse affects every target equally.

 

Hypothetically speaking, I think fourth-year Hemione would fail to kill a full-grown wizard with Avada Kedavra but she could succeed at killing a pig with the same curse. Nearly all spells can be cast at varying levels of strength.

And I agree that different wizards have different killing thresholds, in proportion to their magical power.

 

This really feels like a reach. Extrapolating from some words spoken off-the-cuff to a bunch of school children to a binding rule on the nature and operation of the Killing Curse seems... flawed.

I mean, it works, I guess. If you interpret what Barty says as being completely without artistic license on his part, and do an awful lot of hand-waving regarding the whole basis of Harry's fame. More naturally, I'd say that Barty is simply explaining that no-one in the class would be able to cast the spell. No flashing green light, no swooping sound of onrushing death. The 'not so much as a nosebleed' seems more like figure of speech than an actual statement that the Killing Curse can go flashy green swoopy death and result in a black eye. If that was the case we'd expect it to be a pretty common occurrence that people 'survive' Killing Curses, because people botch spells with a fair amount of regularity.

Sure, you could frame it like that, and completely re-frame Harry's fame as 'He was too magically powerful for Voldemort to kill!', or 'He's the only person Voldemort ever failed to kill with the Killing Curse' (because we still assume people don't know about Lily's sacrificial shenanigans), but that is an explicitly AU element. That's not what he's famous for in canon. In the very same conversation you're referencing, Barty says Harry is the only person to ever survive the Killing Curse, and the way it's stated seems much less ambiguously reinterpretable.

Really, the statement that Harry is the only survivor of the Killing Curse doesn't leave much wiggle room. Consider the Atrium situation where it has been suggested that the Killing Curse aimed at Harry was less powerful than the one pointed at Dumbledore (though that raises the question of why Voldemort would rein it in when trying to kill Harry in this situation, what exactly does he gain from not going all out with every spell, assuming magic is not an exhaustible resource?), and imagine a situation where a powerful wizard jumps in front of a less powerful wizard and shields them using their own body. This doesn't seem entirely unlikely, I imagine people in relationships would do this frequently enough that we'd have some documented instance of failed Killing Curses.

I considered proposing a time-reversed spell effect, where the green light and swooshing death effect is decided post-facto when the spell actually connects. Point wand, spell cast, hits target, check magical power and if < spell power then step back in time and add green light etc so that the only visibly cast, working Killing Curses are the ones which work, but it just doesn't fit with what we see in canon.

 

I always figured that in order for a killing curse to do anything at all, it has to be cast successfully, and took Moody's comment to be hyperbolic. I mean in the sense that any killing curse will kill any human being, though you can of course cast it with different degrees of forcefulness, as we see from the Dumbledore vs. Voldemort duel quoted before.

I don't know that e.g. Bellatrix's use of other spells is indicative of much of anything. For example, it could be that other spells are quicker to cast, thus making the repeated use of the killing curse a tactic of limited effectiveness. We can also see that skilled wizards can block and dodge the killing curse -- as you said, one has to first maneuver people in to a situation where the killing curse could be used.

There's another indication that just spamming the killing curse mindlessly probably doesn't work very well in a fight: in the HBP fight, a blonde death eater (Rowle?) uses the killing curse repeatedly, but fails to kill any order members (and instead manages to accidentally kill one of the death eaters). Granted, some of the people there had used Felix Felicis, but regardless, Rowle's strategy was clearly rather ineffective.

The question about nundus and dragons is interesting, though. It seems to me that it must be the case that the average wizard simply can't kill them with a killing curse. This would either mean that the average wizard can't cast the killing curse (may be true, but surely not related to skill -- we see morons like Crabbe and Goyle managing it) or those magical beasts are capable of withstanding at least most killing curses. I lean towards the second explanation myself, given that Harry, who is not generally prone to cruelty, manages to use two of the three unforgivables.

 

This bit might be explained by Bellatrix wanting to cause torture/suffering instead of instant kills.

EDIT: In fact, in metaphorical sense, Voldemort is the Killing Curse, Bellatrix is the Cruciatus and Barty Crouch Jr. is the Imperius.

 

2. In duels involving powerful dark wizards, why do they use any directly offensive spell other than the Killing Curse?

Perhaps because they're unable to cast it quickly and nonverbally? Having to stop and think of a long incantation like Avada Kedavra would leave you open for a counterspell and shouting it would be even worse. The movies make it seem as if all you need to do is to point your wand and shoot a spell, even while grunting and make unnecessary flourishes with your wand, but I imagine that in canon you really need to concentrate to use nonverbal magic, and while it may be quicker and more discreet than shouting, not everybody would be able to cast the Killing Curse nonverbally any time they want.

I also changed my mind and if you can't resist a simple stunning spell then you also have no defenses against Avada Kedavra, half-giants and other magical beings have natural defenses against curses. But wizards are just humans who can use magic, there are no mundane dragons like there are mundane humans (muggles).

A properly cast killing curse can effectively kill humans and that's it. Dragons, Giants, Manticores, and other creatures must be killed in a different way because they are not humans and have powerful magical defenses.

 

I always took this to mean that no one could cast the spell and that anyone who did cast one would cast one capable of killing humans.

My own speculation regarding the killing curse is based upon magical nature. The nature of a dragon and other magical creatures grants resistance to spells in general. As such, it takes more/better killing curses to kill them. It is a continuation of the ideas with the stunning spells against the dragons and McGonagall. It's immensely difficult for a wizard to kill a dragon with a killing curse, but Voldemort might be able to do it.

I don't think this means one wizard or another would have a magical nature that grants better resistance to the killing curse. The magical nature of every wizard is the same which is why squibs aren't on a spectrum. They simply can't use magic. It's how well someone learns magic that determines a wizard's skill with magic. But skill isn't involved with determining one's survival of a killing curse.

And I think canon does contradict the idea of a skilled wizard simply surviving the killing curse. Harry is treated as being famous for being the only survivor of the killing curse. Surely someone somewhere would have cast a killing curse and failed, leading to another survivor. I think the killing curse is binary when it comes to humans. While I think the ability of the user who casted it matters for magical creatures, I think all killing curses actually cast from a wand (sort of a threshold of sorts) are lethal for humans.

And as for why wizards don't simply just use the killing curse, it's not as if the killing curse can't be defended against. It might not be blockable with a magical shield or a Snape-esque block, but it can be defended against. This means that variety matters in a duel. It would be all too easy to defend if your opponent kept on casting killing curses at you. Add in an animation to attack from the side, a blasting curse, and a killing curse and suddenly it's harder to defend. I do think the killing curse is probably one of the best spells to use in a duel, but I personally don't mind that at all.

As for the unforgivables and intent, I think people have taken Bellatrix's description of the cruciatus and taken it to mean that all the unforgivable's intent has to fall under a similar theme, sadism.

Cruciatus - sadistic intent required. It's why Harry's righteous anger failed and why his vengeance motivation worked better with Carrow.
Imperius - desire to hijack someone's will required. Harry was more fueled by the intent to not get caught, and the imperius was a convenient way to do so. It's why it didn't work perfectly.
Killing curse - intent to kill required. Nothing really special. You just have to intend to kill as you might intend to levitate something with the levitation charm. It's why Bellatrix could nonverbally kill a fox without even knowing it was a fox. It's why Crouch could kill a spider. You need to intend to kill but there's no special sadistic intent required, or some enjoyment to be had.

 

I've mentioned this before, but size can potentially also factor in to how much power is needed for a creature to die from the killing curse. However, the way wizards talk about nothing ever surviving the killing curse makes it seem like there's never been a recognizable casting of the AK that has hit a target and failed to kill it. I would find it strange if a wizard has never cast a recognizable but weak killing curse against a superior opponent at any time in history. Steelbadger's suggestion about having the curse fail to be the recognizable, successfully cast killing curse depending on what target it is going to hit kind of works, but feels a bit inelegant.

I don't think most wizards in canon have the ability to defend against the killing curse in heated battles.

I wonder if a cooldown mechanism might explain why AKs are not spammed by lower level death eaters. It isn't exactly supported by canon, but it might be able to be fit into the system.

 

I feel like Taure's theory would work better if the emphasis on canon was put on no one surviving Voldemort's killing curse rather than no one surviving any recognizably cast killing curse.

The scene in the atrium differentiating the killing curse cast against Harry vs Dumbledore is interesting though, assuming that was a killing curse against Dumbledore.

 

Rowle managed to spam killing curses just fine, to the point of killing his fellow death eater, which I think directly contradicts the idea of a cooldown. The idea of a cooldown mechanism feels distinctly unlike Harry Potter.

 

I'd imagine that as projectiles go, the Killing Curse is relatively lockable. We see it blocked a few times in the series and we know it doesn't work on physical objects, it seems that against any sufficiently skilled Wizard conjuration counters any killing curse. The main caveat is that it is magically unblockable. There's also the matter of how to summon the curse.

We know that it requires an inordinate amount of hate and anger. It can't even be righteous anger or hate, at least, if you want it to work as intended. Emotions like that aren't easy to just "produce."