Complex sentence: The Rock would go on a lengthy feud with Mankind, which is considered to be one of the best feuds of the Attitude Era, battling him in a number of high profile matches, including the infamous "I Quit" match at the 1999 Royal Rumble.
Split into simpler sentences: The Rock would go on a lengthy feud with Mankind, which is considered to be one of the best feuds of the Attitude Era. Battling him in a number of high profile matches, including the infamous "I Quit" match at the 1999 Royal Rumble.