Over the course of seven books and eight movies—with the ongoing Harry Potter fans have seen their favorite characters intuitively pick up some spells while struggling to cast others—and they're not alone in that struggle. Some spells are just harder to cast, plain and simple. They require more experience, more focus, more knowledge, or a particular frame of mind or particularly powerful emotion in order for their casting to succeed.
Schools like Hogwarts haven't earned such prestige in the wizarding community for no reason; graduates are known for their mastery over many aspects of spellcasting, with the following being of particular importance.
Updated Jan. 14, 2022: While many of the things taught to students of Hogwarts School of Witchcraft and Wizardry require incredible spellcasting skill, there are plenty of tough incantations that go unmentioned. From Severus Snape's sordid Sectumsempra spell to the various unforgivable curses frequently used by Voldemort and his Death Eaters, it's important to some hard-to-cast spells about which many in the wizarding community would likely prefer to forget.
Any Non-Verbal Spell
Though it doesn't necessarily pertain to a specific incantation, non-verbal spellcasting is said to be incredibly difficult to master. This is a fact that came up several times in Harry Potter and was showcased brilliantly in the Fantastic Beasts series.
Non-verbal spellcasting gives the a significant edge in battles, obviously, but it's also more practical and safe when casting a spell in a location with muggles around. Yet, despite these advantages, most witches and wizards don't bother mastering the technique, and that's because of the extreme amount of focus required in order to do so. It isn't a skill for everyone.
Obliviate
Obliviate is an extremely difficult spell to cast, and that's probably for the best. Obliviate is a spell used when someone wants to remove a person's memories, and, as such, it really shouldn't be used lightly. However, it has been convenient in keeping muggles unaware of the magic all around them.
Obliviate requires the caster to know what memories need to be removed. It's sort of like using a scalpel instead of a sword; the caster has to know what they're aiming for. That's the main reason why muggles don't hear about people losing their memory all over the place.
Polyjuice Potion
It's not technically a spell, but the Polyjuice Potion is worthy of mention because of its direct importance to the Harry Potter storyline. It was first mentioned in The Chamber of Secrets when Hermione brewed it, and it again appeared in The Goblet of Fire when Barty Crouch Jr. used it to assume Mad-eye Moody's role at Hogwarts.
Said to be extremely difficult and time-consuming, the Polyjuice Potion is among the most advanced wizarding practices mentioned throughout the Harry Potter series. The devastating effects of implementing it incorrectly were also seen when Hermione accidentally turned herself into a cat.
False Memory Charm
The False Memory Charm is both similar and dissimilar to the Obliviate spell. While the latter removes memories, the former replaces unwanted memories with something else. Understandably, this is an extremely difficult spell to cast.
There are only a few instances of this spell being used during the course of the novels. In one of those instances, it was performed by Hermione Granger in an attempt to protect her parents from dark wizards. Those familiar with Hermione's spellcasting skill can get an idea of how much determination and drive this spell would have required.
Undetectable Extension Charm
The Undetectable Extension Charm is another difficult spell to cast. It's also one that has been heavily regulated by the Ministry of Magic, which is telling. The charm does exactly what it sounds like; it expands the area within an object, but it does so in a way that won't be picked up by most detection spells.
The shining example of this spell was done by Hermione Granger, where she put the spell on her purse in order to carry all of the supplies that she, Harry, and Ron would need on their quest. And a spell has got to be difficult when Hermione herself says it was a 'tricky bit of work.'
Sectumsempra
When Harry comes to own a second-hand potions book, he discovers that it's rife with wizarding tips and tricks, and the prior owner even invented some of their own spells. One of which was Sectumsempra, the use of which was said to be reserved "for enemies."
Able to inflict numerous deep lacerations on an opponent's body, Sectumsempra is extremely dangerous and not to be taken lightly. While the difficulty of the spell is never exactly specified, Snape's incredible knowledge of curses indicates that its creation and use were and are not easy. Given the visceral nature of the spell, it's also not for any faint-of-heart wizards.
Apparition and Disapparition
Both Apparition and Disapparition are considered to be extremely difficult spells to successfully cast - at least, at first! In fact, there are special instructors that will come in and teach students the technique, and these specialists not only need to know how to teach it, but how to undo any damage done to the students when it is done wrong.
That is why both spells are forbidden to students underage, as the risk is simply too high. The hope is that older students will have enough discipline to use them safely. The difficulty of the spells is made clear when one realizes that many witches and wizards would simply prefer to travel by other means instead.
Occlumency and Legilimens
Occlumency and Legilimens are two sides of the same coin; one allows the to defend their mind from outsiders, while the other allows them to read the mind of others. It is believed that, of the two, Occlumency is more difficult. That is likely due to the constant vigilance—to borrow a term from Moody—required, along with a variety of other challenges.
Occlumency requires the to completely clear themselves of all thoughts, and that is a trick that most of us wouldn't be able to do. Harry certainly struggled with it, and he had some of the best incentives out there to try and make it work.
Cruciatus Curse
One of the three unforgivable curses deemed morally unacceptable for use by the wizarding community, the Cruciatus Curse allows the caster to inflict torment on a victim, but it requires incredible skill and virulent feelings of malcontent, something of polite society aren't often brimming with.
Voldemort and his gang of Death Eaters make frequent use of the curse to extract information from and punish their enemies. Harry even attempts to cast the spell on Bellatrix Lestrange in Harry Potter and the Order of the Phoenix, but he's only able to knock her off of her feet with it.
Animagus Spell
Sirius Black may make this magic look easy, but he's an experienced Animagus, and things aren't nearly as stress-free for less-experienced spellcasters. The first time a witch or wizard hopes to cast the spell, they must go through an entire process.
The Animagus Spell combines potions and transfiguration, and the preparation is exceedingly complicated. Mes at any one point will require the caster to start over—assuming that they didn't damage or alter themselves in the process. That is the main reason why there are so very few Animagus out in the world.
