Complex sentence: Lincoln Hall was left for dead while descending from the summit of Mount Everest on 25 May 2006, after suffering from a form of altitude sickness, probably cerebral edema, that caused him to hallucinate and become confused.
Split into simpler sentences: Lincoln Hall was left for dead while descending from the summit of Mount Everest on 25 May 2006. He had fallen ill from a form of altitude sickness, probably cerebral edema, that caused him to hallucinate and become confused.