Complex sentence: He joined a notorious guerrilla force known as McNeill's Rangers, a 210 man battalion-size unit that was known for its brazen attacks, unpredictable behavior and relentless raids.
Split into simpler sentences: He joined a widely feared guerrilla force known as McNeill's Rangers. This 210 man battalion-size unit was known for its brazen attacks, unpredictable behavior and relentless raids.