Complex sentence: Obi-Wan retrieves his former friend's lightsaber and returns to the shuttle, leaving his nemesis to die but, unknown to Obi-Wan, survives and Palpatine rebuilds him as the black armor-clad cyborg from the original trilogy.
Split into simpler sentences: Obi-Wan retrieves his former friend's lightsaber and returns to the shuttle, leaving him to die. Unknown to Obi-Wan, his nemesis survives and Palpatine rebuilds him as the black armor-clad cyborg from the original trilogy.