I'm thinking that it was long enough in the past, that only the long-lived (elves) have any actual knowledge of it; they are isolated and keep to themselves (I liked names like Laurelin, Eldamar, and Namarie for the forest-cities of the elves).
My first thoughts about the cataclysm are tied to the magic affliction … the Empire was built using much powerful magic … they even held back the tide … through either arrogance or ignorance, they pissed off the gods of magic and their magic ceased to work, the tides were no longer held back and the oceans crushed through the empire, the saltwater destroying the once fertile lands and creating a massive canyon where they broke through and a series of deserts and horrid wetlands.
How about something more active on the part of the Empire: they unleashed something at the academy south of the capital that sent a shock through the land that raised the mountains, sunk some of the former dry land, etc. It also affected the very nature of magic (thus the magic item use rules).
How about 800 years ago? That would mean humans might still have stories about the Empire but that they are more myths. Would still allow for some of the sunken forests to still have tree stumps :)