Tell the story of the downfall the Reunited Kingdom (i.e. Gondor + Arnor)

I can’t see Sauron having power or tech on that scale. If he had, he’d have used a lesser version of it on one of the lesser cities of Gondor to speed the surrender of Minas Tirth.

I’m sure it will turn out to be an accident. Some scientific expedition(or what passed for one back then) starts poking around the ruins of Barad Dur or Minas Morgul and hits a pocket of Pure Evil or a switch labeled in the Black Speech(which hardly anyone can translate anyway) and…