Complex sentence: They also played two sold-out dates at the 10,000-capacity Monterrey Arena in Monterrey, Mexico--an unusual accomplishment given that Norteño groups typically play large dance halls and rarely arenas unless it's an all day festival event.
Split into simpler sentences: They also played two sold-out dates at the 10,000-capacity Monterrey Arena in Monterrey, Mexico. It was an unusual accomplishment, given that Norteño groups typically play large dance halls and rarely arenas, unless it's an all day festival event.