Salazar Slytherin's locket is one of the seven horcruxes. Remember: Lord Voldemort divided his SOUL into 7 parts to protect himself from death. Each part was attach ed to an object that was particularly important to him. This is what we call an Horcrux. The locket is a piece of JEWELLERY (jewellery = des bijoux / a piece of jewellery = un bijou) that belonged to Salazar Slytherin, whose initials are SS, one of the founders of Hogwarts. Voldemort hid this locket into a CAVE (= grotte). Dumbledore and Harry go there in the sixth book of the series: Harry Potter and the Half-Blood Prince but the locket they find is a fake. The real one is gone. 

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Salazar Slytherin and the house he created at Hogwarts are closely linked to snakes. He even placed a snake in the Chamber of Secrets before he left the school! Slytherin, just like Tom Riddle (Lord Voldemort) and Harry Potter, could speak the language of snakes: Parseltongue. To find the locket, you had to go to the biology laboratory and find the snakes!

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Salazar Slytherin was one of the four founders of Hogwarts, along with Godric Gryffindor, Helga Hufflepuff and Rowena Ravenclaw. He hated taking students from Muggle (non-wizard) families, seeing them as untrustworthy (= non digne de confiance) and unworthy (= indigne) of being taught magic, and he tried to persuade the other founders to only accept students from pure-blood families. However, he failed and he eventually (= finalement) chose to leave the school.

 

His initials are « SS », which are also the initials of a major paramilitary organization under Adolf Hitler and the Nazi Party. This is certainly no coincidence. The author of Harry Potter, J.K. Rowling, probably wanted to establish a link (= a connection) between the Nazi ideology of a master race (they believed that some people, part of what they called the Aryan race, were superior to others) and Salazar Slytherin’s contempt (= mépris) for non-wizard people and his belief in the superiority of « pure-blood » families.