The Harry Potter series tells the story of a young boy who grows up to be a wizard. He is hunted by a man described in the books as the most evil wizard, Lord Voldemort. Voldemort resembles a wizard Hitler with his quest for blood purification, but amazingly, Voldemort isn't the scariest character in the series.
While Voldemort was the big bad, some other Harry Potter characters would do far more evil things in service to the Dark Lord. Any character in the series may have wanted to avoid Voldemort, but they definitely wanted to avoid these servants to Voldemort.
8 Grawp
The half-brother of Hagrid, Grawp, stands 16 feet tall, towering over everyone else readers meet in the series. He is one of the giants who are thought to be vicious by nature, even though Hagrid, being half-giant, and Grawp have proven that untrue. Still, he is a recluse who isn't very smart, which can mean trouble for those who come across him.
What makes Grawp scary is his lack of control. He's clumsy but also feisty. He could accidentally step on someone. He's not villainous, but he's still a scary creature to run into.
7 Basilisk
A giant snake that can only be controlled by the Heir of Slytherin and lives in a secret chamber beneath a girl's bathroom is an evil atrocity that can only be found in the Wizarding world. The basilisk has venomous fangs and can kill someone with just a stare. Fortunately, none of the Hogwarts students saw the basilisk's eye directly, so they were only petrified.
While this evil creature didn't cause nearly as much havoc as Voldemort because only Voldemort could call it from its chamber, either by parseltongue or possessing Ginny Weasley, it still had the potential to be the most dangerous creature and far scarier than Voldemort himself.
6 Nagini
While not as big or as powerful as the basilisk, Nagini may be a more useful evil magical creature for Voldemort to use. Nagini is huge in her own right but is still small enough to actually work to kill people on her master's command. Plus, Voldemort used Nagini's milk to stay alive until he returns to full power.
She is able to kill people with a bite and was also able to live inside the body of Bathilda Bagshot when Voldemort was setting a trap for Harry Potter. Nagini also serves as one of Voldemort's horcruxes making her even scarier since she contains part of the Dark Lord.
5 Aragog
Spiders are frightening. It's why arachnophobia is one of the most common fears in the world. But that fear is ratcheted up the scales with the introduction of the acromantula. These giant spiders feast on human flesh if someone comes across them. Aragog avoided eating humans do to respect for Hagrid, but he wouldn't stop his kids from eating humans in the Forbidden Forest.
While spiders may fear the basilisk, many people around the world would have an easier time handling snakes than spiders. Aragog may have given Harry and Ron answers to the questions they were asking, but he was certainly one of the most visually frightening creatures in the entire series.
4 Dementors
Cloaked creatures that can take away every feeling of happiness before sucking your soul from your body, the dementors were introduced as the guards of Azkaban prison who were searching for Sirius Black at Hogwarts. They are completely blind and have no loyalty to anyone. They just want to suck people's souls.
J.K. Rowling has admitted to coming up with the idea for dementors while batting a bout of depression. These creatures make their victims relive their worst moments before leaving them in a state worse than death.
3 Fenrir Greyback
The most evil people in history usually had a limit to what they would do themselves, instead handing over some of the dirtier tasks to their sycophantic followers. This perfectly describes Fenrir Greyback. A werewolf who is constantly angry and wants to turn others into werewolves, especially children.
Mistreated by wizards, he was quick to join Voldemort's cause because the Dark Lord treated him marginally better. He was responsible for Remus Lupin becoming a werewolf. He attacked Bill Weasley leaving him scarred. Greyback was ready to attack anyone or anything in his way.
2 Bellatrix Lestrange
While Voldemort kills for power, Bellatrix Lestrange loves to kill for fun. She is fiercely loyal to the Dark Lord and his cause. She didn't try to defend herself when prosecuted for her crimes. She wore her evil deeds proudly and served her time in prison without remorse.
Bellatrix Lestrange killed her own cousin and tried to kill her niece. She was also one of four Death Eaters who tortured Frank and Alice Longbottom to the point of insanity. Death would've been better for the Longbottoms and their family.
1 Dolores Umbridge
A middle-aged woman who looks like a toad and seems to have a moral code that guides her life doesn't seem that frightening at first. However, Dolores Umbridge is the type of government official who tries to force her morals on others and is willing to use extreme measures of punishment to get her point across.
She has power given to her by the people, and she abuses it. She won't directly kill you, but she will try to frame you for her own personal gain, as she did with Harry Potter and the dementors in a muggle neighborhood. In a world of fantastical creatures and magic, she is the scariest evil there is, the one you might meet in the real world.