Sos the Rope — Piers Anthony

Earth's demon. They call him Sos - a man mightier in strength and spirit than the greatest warriors of post-Blast legend. His past is shrouded in enigma, but now he wanders the earth, writing a heritage in blood and bravery.
To some, he is savior; to others, agent of cataclysm. One man alone bears the terrible curse of his friendship. Two mortal women share his doom-driven love.
Yet only Sos knows the ultimate purpose of his strange, violent mission on earth: save mankind - or destroy it forever.