Complex sentence: The Rancor is an immense, warm-blooded predator native to Dathomir, Lehon, and Felucia; but first seen in "Return of the Jedi" wherein Luke Skywalker is attacked by one at the behest of Jabba the Hutt.
Split into simpler sentences: The Rancor is an immense, warm-blooded predator native to Dathomir, Lehon, and Felucia. It was first seen in "Return of the Jedi" wherein Luke Skywalker is attacked by one at the behest of Jabba the Hutt.