Complex sentence: Luke and Vader engage in a lightsaber duel that leads them over the city's central air shaft where, as his mentors warned, Luke proves to be no match for Vader who severs Luke's right hand, causing him to lose his weapon.
Split into simpler sentences: Luke and Vader engage in a lightsaber duel that leads them over the city's central air shaft. As his mentors had warned, Luke is no match for Vader who severs his right hand, causing him to lose his weapon.