Simplify the following text for a Grade 9 native English-speaking person.
Original: A new character appears when former Hogwarts Potions master Horace Slughorn replaces Snape , who assumes the Defense Against the Dark Arts post .
Simplified: A new character appears when former Hogwarts Potions master Horace Slughorn returns to replace Severus Snape , who takes over the Defense Against the Dark Arts post .