Tom Riddle has turned into 'Voldemort' after having Jughead. A spirit named Voldemort possed Tom's body.
There are eight Horocruxes in total and Jughead is one of them. One of your first questions might be "How the heck is Jungkook a Horcrux if he had the time to kill him before he became Voldemort?"
Well, a second before he was fully transformed he told Lilly (Jughead's mother and Tom's wife) to take Jughead to the Andrew's (they lived next door). When he got there he was fully transformed and Tom had no control over Voldemort's actions and killed Sierra, Lilly, and Natalie .
But Lily and Sierra had protected baby Jughead and when Voldemort tried to kill him, he failed.
That was the moment Jughead got his scar. (The only reason Voldemort was kind to Jughead was Tom's love was so strong for him it passed onto Voldemort.)
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