<P> At the start - of - term feast at Hogwarts, Dumbledore announces Snape as the new Defence Against the Dark Arts professor . Horace Slughorn, a retired Hogwarts teacher, replaces Snape as Potions Master . With Snape no longer teaching Potions, Harry enrolls in Slughorn's class and is lent an old textbook until his new one arrives . Harry finds marginalia, including a variety of hexes and jinxes seemingly invented by an unknown student, and substantial improvements to the book's standard potion - making instructions . The text is inscribed as being "the Property of the Half - Blood Prince". The notes greatly bolster Harry's performance in Potions, so much so that he impresses Slughorn . Snape, who maintains that he "never had the impression that (he) had been able to teach Potter anything at all", is suspicious of Harry's newfound Potions success . </P> <P> Later, during a fight with Draco, Harry casts one of the Prince's spells marked "For Enemies," and is horrified by the devastating wounds it inflicts to Draco's face and chest . Snape rushes to the scene and heals Draco, then interrogates Harry regarding the spell, using Legilimency to extract the source of Harry's knowledge (the Potions textbook) from Harry's mind . When Snape insists that Harry show him his Potions textbook, Harry hides the Prince's book and gives him Ron Weasley's book instead . As punishment for the attack and knowing Harry is lying about the textbook, Snape assigns Harry detention during the final Quidditch match of the year . </P> <P> Before leaving Hogwarts to accompany Dumbledore in locating another horcrux--part of Voldemort's soul--Harry discovers from Professor Trelawney that it was Snape who overheard the prophecy and told it to Voldemort, resulting in Voldemort hunting down Harry and his parents . Despite this and Harry's angry questions, Dumbledore avers his trust in Snape . Returning to Hogwarts after retrieving Voldemort's Horcrux, Harry and Dumbledore alight atop the school's astronomy tower . Gravely weakened by the horcrux's protective potion, Dumbledore tells Harry he must fetch Snape, but before Harry can leave, Draco suddenly arrives, intending to carry out Voldemort's order to assassinate Dumbledore, closely followed by other Death Eaters (followers of Voldemort), and Snape . Snape interrupts the planned murder, killing the headmaster himself . Harry, who is paralysed under his invisibility cloak by Dumbledore for his own protection, witnesses the kill, and is released upon Dumbledore's death . Enraged, he pursues Snape, Draco, and the Death Eaters as they flee the castle . Snape easily blocks Harry's spells and jeeringly points out Harry's mistakes, but never strikes back . During the confrontation, Snape reveals himself as the eponymous "Half - Blood Prince" (being the half - blood son of Muggle Tobias Snape and pure - blood Eileen Prince). Snape passes through the school gates and Disapparates with Draco in tow at the book's end . The full relationship between Dumbledore and Snape and the reason for Snape's actions remain unknown until the final book . In an interview, Rowling mentioned that at this point in the series, the Harry--Snape relationship has become "as personal, if not more so, than Harry--Voldemort ." </P> <P> In Harry Potter and the Deathly Hallows, Voldemort and his Death Eaters have tightened their grip on the wizarding world . Snape is named Headmaster of Hogwarts, while Death Eaters Alecto and Amycus Carrow are appointed as Hogwarts staff . The novel focuses largely on Harry and his friends and events outside the school; therefore Snape plays a limited role at the start . In the course of the book, Harry and his friends find out that a few students attempted to steal the Sword of Godric Gryffindor, and that Snape subsequently sent it to be stored at Gringotts Wizarding Bank; however, that sword was only a copy . Later on, Harry and Ron are led to find the real sword by a Patronus taking the form of a doe . (Towards the end of the book, Harry learns that this was Snape's Patronus, taking the same shape as Harry's mother Lily's Patronus, a visible sign of his lifelong love for Harry's mother, and that Dumbledore had asked Snape to ensure that Harry gained possession of the sword . The novel also reveals that Snape had covertly used his position as Headmaster to protect the students and to contain the Carrows .) </P>

Why did snape call himself the half blood prince