Summary:

Harry Potter has done it: he has defeated the Dark Lord. Yet this victory has come at a great price: namely, the eradication of the entire human race. Wizards, muggles, and creatures alike perished and now Harry is alone left, cursed with the title of the Master of Death and an extended life. So, Harry does what any wizard with too much time and too little common sense would do: he goes "traveling".

Harry Potter and Lord of the Rings crossover.