It's been six years since the last Harry Potter film hit theaters, but the series still remains one of the most successful and beloved franchises ever created. J.K. Rowling's exploration of the Wizarding World was chock full of imaginative creatures -- friendly ghosts that roamed the halls of Hogwarts, patronuses that protect against forces of darkness, and fierce phoenixes that always knew when to come to the rescue.

Many of the author's creations, though, were darker and far more frightening. From werewolves to immortal dark lords, the villains that Harry Potter faced throughout the series were considerably dark, especially given that the series was geared toward children. Some were monstrous because of their appearance; others looked relatively non-threatening but carried dark and evil plans to destroy the Boy Who Lived. All of them made Harry's time at Hogwarts incredibly complicated. Here's our take on The 15 Most Terrifying Villains In Harry Potter.

15. The Mermaids

If you grew up in a pre-Harry Potter world, chances are your concept of mermaids were more in the sings-pretty-songs-and-brushes-her-hair-with-a-fork vein. These subaqueous creatures took on a decidedly more terrifying public perception after they cropped up in Harry Potter and the Goblet of Fire. They only appear briefly, during the second task of the Triwizard Tournament, but it's enough to instill a Jaws-like fear of deep water.

Harry, Cho Chang, Viktor Krum and Cedric Diggory meet the mermaids when they dive into the Black Lake to retrieve something (er, someone) very important to them. The mermaids lure them toward their prizes with a beautiful, haunting song before attacking them with pitchforks. Yeah, their vocal stylings were right on par with Ariel -- but their creepy, sunken faces, Gollum-esque eyes, pointy teeth and willingness to kill Harry and the other Triwizard Tournament competitors was decidedly not part of a happy-go-lucky world.

14. Argus Filch

Some of the villains in the Harry Potter universe are scary because they're downright evil. Argus Filch, Hogwart's caretaker, is creepy for an entirely different reason. Throughout most of the series, he skulked around in the background looking every bit the part of a villain. From his long, stringy hair and the permanent grimace on his face to the glee he seemed to take whenever a student was in trouble, Filch wasn't ever a warm or friendly presence. Though Hogwarts served as a (mostly) safe haven for our favorite Gryffindors, Filch's mean-spirited aura served as a constant reminder that even the most sacred of spaces could quickly turn bad.

That was especially true when he teamed up with Professor Umbridge in Harry Potter and the Order of the Phoenix and relished in dishing out punishment to any student that stepped out of line. The fact that he sought out ways to catch Harry and his friends breaking school rules made him a menace during their schooldays, even if he was never a genuine threat to their existences.

13. The Basilisk

The ability to talk to snakes might sound pretty cool on paper, but when it lead Harry to the Chamber of Secrets, it turned out to be a skill with some serious downsides. What he faced there wasn't just a glimpse at the angsty teen that would be Voldemort, but something more immediately terrifying. Enter the Basilisk, a larger-than-life fanged beast that was all-too-happy to chow down on the young Mr. Potter. It stalked Harry through its subterranean Hogwarts hideout, shrieking and drooling and generally proving to be a pretty terrifying menace to the 12-year-old Chosen One. Seriously, it's a pretty ferocious beast for a second year magician-in-training to have to fight off -- thanks Tom Riddle.

Only a combination of dumb luck and a little help from Dumbledore's phoenix, Fawkes, helped Harry and an unconscious Ginny escape certain death. The Basilisk wasn't as harmful as many of the other Harry Potter villains -- but it definitely proved to be one of the scariest creatures that the young wizard ever encountered.

12. Peter Pettigrew

In Harry Potter and the Prisoner of Azkaban, we learn some pretty interesting info about Harry's dad -- like that he could change into a stag, and all his Hogwarts BFFs were animagi, too. Unfortunately, one friend, Peter Pettigrew, was so evil that he teamed up with Voldemort, revealed the Potters' location to them, and ultimately aided in their death. Pettigrew then did what any honorable Death Eater would do: he turned himself into a rat (too obvious? Maybe.) and passed himself off as Ron Weasley's beloved pet Scabbers.

What made Peter so scary wasn't just his incredibly rat-like appearance when he finally revealed his true identity. It was his complete willingness to be a mole for Voldemort back when the original Order of the Phoenix was working so hard to defeat him. His cowardice allowed for Sirius Black to spend more than a decade in Azkaban, too. Peter -- a friend and ally turned subservient villain -- serves as a cautionary tale for us all that sometimes we keep our enemies too close without even realizing it.

11. Tom Riddle

If we learned anything from the Harry Potter series, it's that it takes a Hogsmeade-sized village to build a supervillain. We saw, in both The Chamber of Secrets and The Half-Blood Prince that Tom Riddle didn't transform into Voldemort overnight. Yes, he had a lot of willing help in carrying out his dark orders, but he also used his powers of manipulation to get crucial information about immortality from his seemingly harmless Hogwarts professors.

The ease with which he-who-would-become-He-Who-Must-Not-Be-Named rose to power was chilling. Even scarier, though, were the glimpses we got of the young Tom Riddle as he began to somberly and quietly put the pieces of his master plan together. He had no qualms about hurting others or using them to get what he needed. He seemed to relish in using his intelligence and charisma to get the upper hand. Riddle is frightening because he reminds us how easily true evil can fly under the radar.

10. Nagini

Thanks to Harry's penchant for parseltongue and the serpentine symbol for Slytherin, snakes played an undeniably huge role in Harry Potter. The latter creature, Nagini, was a particularly terrifying piece of the end game puzzle, though. The huge snake slithered dutifully by her master's side, eagerly chowing down on the Dark Lord's leftovers, be they harmless muggles or even his own allies. In one of the most disturbing sequences in the entire series, she took up residence in reclusive historian Batildtha Bagshot in an attempt to lure Harry and Hermione into a deathly trap.

Nagini wasn't just scary because she was a huge snake -- after all, we already have one of those on our list. It turned out her allegiance to Voldemort was so strong because she was actually a part of him -- one of seven horcruxes meant to contain a piece of his soul and insure his immortality. There's something extra creepy about such a creature existing entirely to keep the darkest wizard ever alive, which is why her death, at the hands of perpetual underdog Neville Longbottom, felt all the more satisfying.

9. Aragog

As the Harry Potter series developed, the books and movies got longer, and the creatures got more frightening. That doesn't mean that the earliest installments were without a few scares, though. In fact, one of the creepiest sequences in the whole series came in The Chamber of Secrets, when Harry and Ron traveled into the Forbidden Forest. There, they encountered Hagrid's beastly BFF Aragog, a huge and ancient spider. He did give the duo answers to their questions about the Chamber, so it's hard to classify him as a pure villain. However, the fact that he promptly set his army of hungry spider babies on the two young wizards definitely puts him firmly in the 'legitimate threat' category.

For any arachnophobe, Aragog's appearance in CoS alone was probably enough to induce nightmares. But the subsequent moments, in which his spawn descended from tree limbs and scuttled across the forest floor, was enough to cause long-lasting trauma.

8. Lucius Malfoy

Dad. Death Eater. Douchebag. We didn't get to spend all that much time with Lucius Malfoy during his brief but unforgettable appearances in Harry Potter, but it was easy to glean the major pieces of his personality. Draco's father had a foreboding presence from his very first appearance, and not just because of the perma-scowl he wore every time he had to interact with common folk. Early in the series, he just seemed like an entitled jerk -- one who looked down on anyone who wasn't a pureblood wizard. Over time, though, it became obvious that he was still very much fighting on the side of pure evil. He was so threatened by the concept of yielding his political power that he was willing to use any means to help bring the Dark Lord back from the dead.

His methods included, in no particular order: using a First Year student to try to open the Chamber of Secrets, disrupting the Quidditch World Cup to send a message that the Death Eaters were back on the prowl, and offering up his son to Voldemort as a vessel with which to kill Dumbledore. His actions throughout the series weren't just unconscionable, they were a chilling example of how power can corrupt and breed generations of hatred.

7. Professor Quirrell

At first glance, Hogwart's stuttering Defense Against the Dark Arts professor was anything but frightening. Still, all Harry Potter fans know that when it comes to the series' characters, appearances can be extremely deceiving. In The Sorcerer's Stone, Harry and his friends spent most of their time trying to prove that someone in the castle -- probably Snape -- was up to no good. They were partially right, but the young Potter learned that it wasn't his Potions master after all, but Professor Quirrell, who was hiding something seriously insidious under his turban.

When Harry met up with Quirrell, he learned that the DADA instructor had been sharing his head with a shriveled, weakened version of Voldemort. Upon unwrapping his headdress, Harry was treated to a particularly terrifying image -- his parents' murderer's snarling face, staring back at him through the mirror. Though his encounter with real Quirrell's two-faced villainy was brief, it was still a shock to both the young boy and those of us who don't expect to see things that scary in a PG movie.

6. Fenrir Greyback

The Harry Potter series introduced countless magical creatures into the fray -- some devised by J.K. Rowling, and some that are deeply steeped in supernatural lore. Her take on werewolves didn't exactly break the mold -- in fact, one of the werewolves we met, Remus Lupin, was a pretty great guy. Once we met Fenrir Greyback, though, we got a decidedly darker take on lycanthropy.

Greyback didn't get much facetime in the cinematic adaptations of her stories, but whenever he showed up, he was good for a fright. He was a gleeful member of Voldemort's squad, happy to tear through the Burrow with Bellatrix in The Half-Blood Prince and hunt Harry, Ron and Hermione during The Deathly Hallows. It wasn't just his actions that made him so much of a menace though. With his hulking build, sharp teeth and soulless eyes, Greyback was definitely one of the most aesthetically unnerving enemies in Rowling's elaborate world.

5. The Death Eaters

History teaches us that evil can't happen without a whole group of people willing to spread it. In the case of Harry Potter, Voldemort's attempt to cleanse the world of Muggles and Mudbloods would never have even gotten off the ground if it weren't for his army of Death Eaters. The witches and wizards that pledged allegiance to the Dark Lord were branded with his Dark Mark -- a magical tattoo that allowed him to call them to his side whenever he needed them.

There were some pretty prominent Death Eaters throughout the series -- wizards that appeared to be upstanding members of society by day, and who carried out Voldemort's dastardly plans in the dark of night. Many of them simply went underground when they needed to, hiding the darker parts of their personality and pretending all was well, or claim that they'd been under Voldemort's sway to escape prosecution if they were caught. Ultimately, that's what makes the Death Eaters so terrifying: they could easily hide their evil from the world, which made it all the more powerful.

4. Dolores Umbridge

At first, this simpering, stringent Ministry of Magic official and Defense Against the Dark Arts instructor was just annoying. In the end, she turned out to be one of the scariest foes that Harry Potter ever faced. In The Order of the Phoenix, Dolores Umbridge was sent to Hogwarts to make sure the Ministry still had control over the school -- and its message that all was right in the Wizarding world. Her strict methods of reinforcing that idea were uncommon at the school, but her methods of dealing with Harry, who insisted Voldemort had returned, were especially evil.

Umbridge relished in punishing the Boy Who Lived, making him use a quill that carved bloody letters into his hand. Her insistence that he was a liar and a fraud was especially heinous because she didn't just speak for herself, but for the entire governing body she represented. The fact that the horrific treatment of Harry came in the form of a seemingly sweet woman with a love of kittens and the color pink made her especially macabre.

3. Bellatrix Lestrange

Bellatrix Lestrange murdered Sirius Black. She tortured Hermione -- and countless other characters, including Neville's parents -- for fun. The fact that her hobbies also include killing harmless house elves and ruthlessly hunting down Harry, Ron and Hermione is just the icing on her completely crazy cake. For years, Bellatrix served as Voldemort's right-hand woman, dishing out the Unforgivable Curses to anyone who stood in the way of his most evil plans. Once she broke out of the wizarding prison Azkaban to rejoin her master, she posed a serious threat to Harry and anyone else who opposed the Dark Lord.

It wasn't just Bellatrix's actions, though, that made her such a terrifying presence in the Harry Potter series. It was the joy with which she took on her evil tasks. Unlike other villains, who held a stoic calm as they went about their deviousness, Bellatrix loved to maim and murder. While many Death Eaters faded to the background after their leader disappeared, she remained staunchly devoted to the Dark Lord until her dying breath.

2. Voldemort

To some, he was known as He-Who-Must-Not-Be-Named. To others with more evil inclinations, he was the exalted Dark Lord. Across the Wizarding World, Voldemort's magical prowess and penchant for evil were so feared by most witches and wizards that they spoke of him only in whispers, even when he was long-assumed to be dead. In exchange for his tremendous power, he lost his humanity -- and his nose -- in the process. Some would argue he never had the former in the first place.

Voldemort isn't just scary because of his snake-like appearance. His thirst for power and hatred of Mudbloods made him a menace to all who opposed him. His willingness to kill anyone who stood in his way, including an infant Harry Potter, terrorized the magical community for years. The depth that he was willing to sink to, though, to achieve immortality, surpasses that of your average everyday Harry Potter villain. The dude split his soul into seven different pieces, just so he'd be harder to kill, after all. That act, in and of itself, is one of -- if not the most -- dark and twisted parts of the entire franchise. Also, did we mention his nose?

1. The Dementors

These robed, evil beings couldn't Avada Kedavra you, even if they wanted to. What they can do is probably even worse, though: they can take your very soul. The Dementors first appeared in Harry Potter and the Prisoner of Azkaban, and immediately proved to be some of the most chilling creatures in the entire Potterverse. With their faceless facades and their lampry-like mouths, they're definitely not the type you'd want to encounter in a dark alley.

But it's their evil magical abilities that make them truly terrifying. Their very presence wipes out any feeling of warmth or happiness out of their immediate surroundings -- that's why they were chosen to guard the Wizarding prison. They're not just a source of buzzkill, though. The Dementors can give their victims a kiss, and when they do, they end up sucking out their souls in the process. J.K. Rowling has said that she created the Dementors as an allegory for what depression feels like. Given the long-term impact of their most disturbing abilities, they're easily the scariest creatures that the author ever created.

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Which Harry Potter villain scares you the most? Let us know in the comments!