Alecto Carrow

Neville Longbottom: " It's been… Well, it's not really like Hogwarts anymore. Do you know about the Carrows?"

Harry Potter: " Those two Death Eaters who teach here?"

Neville Longbottom: " They do more than teach. They're in charge of all discipline. They like punishment, the Carrows."

Harry Potter: " Like Umbridge?"

Neville Longbottom: " Nah, they make her look tame. The other teachers are all supposed to refer us to the Carrows if we do anything wrong. They don't, though, if they can avoid it. You can tell they all hate them as much as we do. […]  I got this one for asking [Alecto]  how much Muggle blood she and her brother have got."
 * — Neville Longbottom tells Harry Potter about the Carrows, and his own recently acquired scars.

Amycus and Alecto Carrow are a brother and sister pair of villains in the service of the series' Big Bad Voldemort. Introduced in book six of the series, they became teachers and Deputy Headmasters at Hogwarts School of Witchcraft and Wizardry by book seven and were known for their singularly cruel punishments and strong anti - Muggle prejudice by students.

Early Years
The Carrows were born sometime before 1981 into a Pure or Halfblood family in Britain, most likely having attended Hogwarts in their youth. They became Death Eaters during the First Wizarding War, yet were among the majority of Death Eaters who did not search for Voldemort after his downfall, believing him dead. However, they managed to avoid being imprisoned in Azkaban in the time before his return, and were presumed to have been present during Voldemort's comeback in Little Hangleton cemetery, as their absences were not noted.

The Second Wizarding War
Amycus and Alecto became more prominent characters when they were introduced as Death Eaters in the Battle of the Astronomy Tower, cornering Albus Dumbledore at the top of the observatory along with Yaxley, Fenrir Greyback, Draco Malfoy and Severus Snape. Both were excited at the prospect of Dumbledore's death, jeering at the headmaster and hooting wildly as they witnessed Snape deliver the curse that ended his life. Their excitement ended rather quickly, as they were faced with members of the Order of the Phoenix and Dumbledore's Army who dueled them ferociously and sent them running for their lives. Harry Potter managed to trip them up as they made for the school gates, but intervention on behalf of Snape allowed them to escape Hogwarts' grounds.

Some weeks later, they, along with the rest of the core members of the Death Eaters' inner circle, attended a meeting concerning plans to take over Hogwarts and the Ministry of Magic at Malfoy Manor. They contributed little to the meeting, other than to join in Voldemort's mocking of the Malfoys and voicing their scorn at Charity Burbage, the Muggle Studies teacher at Hogwarts who preached acceptance and love towards Muggle-borns (she eventually became Nagini's dinner).

Teaching at Hogwarts
Following the Death Eater's takeover of the Ministry and Snape's assumption of the title of Headmaster of Hogwarts, they were appointed Defense Against the Dark Arts and the Muggle Studies teachers (respectively), as well as Deputy Headmasters. Their subject curricula were changed drastically under their guidance: Alecto now began teaching the Dark Arts, instructing students how to perform dark magic such as the Cruciatus Curse, Fiendfyre and the dreaded Avada Kedavra, while Alecto now began to preach to her students about the supposed stupidity and impurity of Muggle and Muggle-borns, the threat they presented to the Wizarding world, and the need to restore the "natural order" instead of covering aspects of Muggle culture and science in class.