Complex sentence: With Clark's help he was able to extricate himself, and when they returned to the stairwell, the others were already gone, so Clark and Praimnath elected to descend instead of heading upward after them.
Split into simpler sentences: With Clark's help he was able to extricate himself. When they returned to the stairwell, the others were already gone, so Clark and Praimnath elected to descend instead of heading upward after them.