Complex sentence: Reacher mentions how the rough stitch work from his existing scar helped decrease the severity of his most recent attack, where a deep, serious gash led to Reacher passing out from blood loss.
Split into simpler sentences: Reacher mentions how the rough stitch work from his existing scar helped decrease the severity of his most recent attack. The cut did produce a deep, serious gash that led to Reacher passing out from blood loss.