[ warning : following are discussion on rumors, which some might find un-Islamic, avoid on your own accord ]
Muhammad bin Nayef was the real crown prince in KSA. That was deliberately put there as a condition for King Salman's crowning, which he accepted.
And once he was throned, he replaced bin Nayef with his own son MbS as the crown prince.
That was the history. Now news is that Nayef is either in critical condition medically or has been arrested, or both.
Watching.