The final empire

A thousand years ago, the prophesied hero from lore rose up to overthrow a great and terrible evil. Only, he lost, and the Dark Lord took over and has been ruling with an iron fist for a thousand years. Ash falls from the sky in this barren land, and mists come every night, deep and mysterious. In this setting, a gang of thieves decides that the prophecies were all lies and that they can't trust in some fabled hero to save them. They decide to take matters into their own hands, and plan a daring heist of the dark lord himself, planning to use the emperor's own wealth to bribe his armies away from him and take over the empire.