Almost any epic level threat will do, but he/she/it should arise as a direct/indirect result of player character meddling (thereby making them responsible to destroy it). Preferable if PCs have a choice in what they awaken, but don't realize it. (think primordials, demon lords, gods, etc...) (it's only railroading in the sense that they'll probably have to fight this baddie towards the end of the campaign).