In the sixth Harry Potter movie - The Half-Blood Prince, one of Hogwarts' most talented, and the smart-mouth, Minerva McGonagall commands Harry Potter and Ron Weasley to take Potions lessons with the newest member of the staff, Horace Slughorn. Professor Slughorn, who resumes the Potions Master post in the sixth Harry Potter movie is a jovial person, unlike Hogwarts' previous Potions teacher, Severus Snape. He welcomes the duo to his class and promises to sort out Ron who claims to be bad at Potions. He orders the boys to fetch their Potions textbook from the cupboard and proceeds with his lecture. Harry's luck is in as he happens to pick up a tatty but annotated Advanced Potion-Making copy.
Harry's annotated copy says it is the property of the Half-Blood Prince, and it helps him excel in Horace Slughorn's class. For example, when the Draught of Living Death recipe instructs to cut up one Sopophorous bean, the Half-Blood Prince's annotation adds, "Crush with blade" to release "juice better." As so, Harry does exactly what the annotations say and brews a perfect Draught of Living Death potion that Horace Slughorn comments would fulfill its purpose. He receives one vial of Felix Felicis as a reward for brewing the best potion. Harry's shenanigans don't end here, and as The Half-Blood Prince progresses, his fascination with the book, and the usage of a dark curse recorded inside it, lands him in trouble.
Harry Uses Sectumsempra on Draco Malfoy
The Half-Blood Prince's copy continues to intrigue Harry. He comes across a spell by the name of "Sectumsempra" and shows it to Hermione asking if she's heard about it. She tells him off saying a self-respecting person should hand the copy in. Yet, he continues to study the fully-annotated book for weeks, develops an obsession with it and shows no desire to find out about the Half-Blood Prince. Harry loves being top of the Potions class, and the brainiac, Miss Granger feels a pang of jealousy. When he refuses to let her take a look at the book, Ginny Weasley snatches it from his hand and announces it belongs to the Half-Blood Prince. While Harry maintains he's curious to know who the previous owner of this copy was, his actions say otherwise. Later, Hermione admits she's visited the library to find in vain, a reference to the Half-Blood Prince.
Harry Potter and the Half-Blood Prince dedicates chapter 24 "Sectumsempra" to the said spell. Harry sees the titular spell is captioned "For Enemies." While he still doesn't know its use, he plans to use it on McLaggen the next time he came up behind him suddenly or abruptly. It is pertinent to mention that Harry only knew Sectumsempra was for enemies and that to him, was reason enough to use it in his duel with Draco Malfoy that took place in the boys' bathroom. Draco fell on the waterlogged floor when Harry wildly flicked his wand at him, yelling, "SECTUMSEMPRA!" His face and chest bled profusely, and it looked like he had been slain with an invisible sword. His chest was soaked in blood and Harry was horrified at what he had done. Moaning Myrtle who witnessed the whole duel, screamed loudly and Severus Snape burst through the door. The professor drew his wand, and muttered an incantation that "sounded almost like a song." He performed this counter-curse three times and with that, Draco was lifted into a standing position. Snape informed him he still needed to go to the hospital, and ordered Harry to stay behind.
Snape Invented Sectumsempra
Sectumsempra curse lacerates the target and leads to severe hemorrhaging. In J. K. Rowling's Harry Potter and The Half-Blood Prince, Harry maintained he didn't intend to wound Draco mortally and reiterated he didn't know what the spell did. After Snape was done interrogating Harry, he gave him detentions with him every Saturday until the end of the term. Hermione called into question the integrity of the Prince person, and Ginny Weasley supported Harry's act as a self-defense measure against the Unforgivable "Cruci-" aka Cruciatus Curse that Draco tried to use on him. She was glad Harry had something good up his sleeve while Hermione was furious at where the spell's use had landed him.
As the sixth Harry Potter movie - The Half-Blood Prince nears the end, Snape reveals he is the Half-Blood Prince. The revelation takes place after he kills Dumbledore using Avada Kedavra, and makes an escape from the castle with Bellatrix, Draco, and others. Harry does not know Dumbledore had planned his death with Snape. Naturally, furious, and devastated, he chased after him and confronted him right in front of Hagrid's Hut. He used the Incarcerous spell on Snape and urged him to fight back, but Snape blocked it.
Harry, gathered strength, got up again, and yelled, "Sectumsempra!" Once again, Snape blocked his hex, moved nearer to him, and morosely revealed, "You dare use my own spells against me, Potter? Yes. I'm the Half-Blood Prince." It turns out, the personally-annotated Advanced Potion-Making copy belonged to Severus Snape when he was a student at Hogwarts School of Witchcraft and Wizardry. Moreover, Sectumsempra was invented by him with the purpose of using it on his enemies such as the Marauders who bullied him at Hogwarts.