Complex sentence: His death was subsequently written into the final season of the show, with his character Leo McGarry died of a heart attack on election night, and has already suffered a life-threatening heart attack in the sixth season episode "The Birnam Wood".
Split into simpler sentences: His death was subsequently written into the final season of the show, with his character Leo McGarry died of a heart attack on election night. His character had already suffered a life-threatening heart attack in the sixth season episode "The Birnam Wood".