Complex sentence: Cena then began a feud with CM Punk, who was leaving the company at Money in the Bank, where he defeated Cena to win the WWE Championship, and left the company with the title.
Split into simpler sentences: Cena then began a feud with CM Punk, who was leaving the company at Money in the Bank. At Money in the Bank, CM Punk defeated Cena to win the WWE Championship, and left the company with the title.