Evidence

Evidence is a short Lord of the Rings / Discworld crossover fanfiction by Camwyn.

The core concept is that at the defeat of Morgoth at the end of the First Age, Samuel Vimes of Ankh-Morpork's Watch is called in to apprehend said miscreant, which he does with his usual competence, attention to detail and getting up of everyone's nose due to his bloodyminded obsession with doing his job.

Evidence on lotrfanfiction.com