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T'Challa must fight any challengers to be accepted as King of Wakanda after his father dies. After that, he becomes the Black Panther.
Several scenes in the 2018 movie, Black Panther, show he already has the super-strength and super-speed of the Black Panther before the challenge.
Why was he allowed to accept the plant juice that transforms his body before he became king?
I can accept answers from other Black Panther material besides the movie as long as the source is canon.