Simplify the following text for a Grade 9 native English-speaking person.
Original: Oedipus becomes the ruler of Thebes after solving the riddle of the sphinx and in the process , marries the widowed Queen , his mother Jocasta .
Simplified: He becomes the ruler of Thebes after solving the riddle of the sphinx and in the process , marries the widowed Queen , his mother Jocasta .