Not a bad start. Have to agree with some of the others though about the "Cataclysm" being an overused
trope.
It has its benefits if you're doing an RPG: you can describe potent artifacts from Before The Fall, and allow GMs to decide which ones are gone and which are merely "lost".
But for storytelling purposes...meh. It's not really necessary. Most historical empires have fallen without any major assistance from Mother Nature. Most common reasons are civil disorder, disease, famine and in some cases collapsing under the sheer size of the imperial bureaucracy. One problem with large empires, particularly in an era before advanced communication, is that they're incredibly hard to manage from the homeland. More and more power has to be delegated to the local governors/viceroys/prefects, to the point where they become pseudo-kingdoms in their own right. Eventually they resent paying taxes to a far-away capital, especially if the Emperor is unresponsive to their needs.
Additionally, creating an ever-larger bureaucracy to oversee the empire is expensive. Creating an army to defend it is also expensive. It's possible to get caught in a conquest death spiral, where the administrative costs of the empire can only be funded through conquest and plunder, and the conquest necessitates an even larger administration and army, which requires more money,
ad nauseum.
There's a Chinese proverb,
shan gao huang di yuan, which roughly translates to "The mountains are high and the Emperor is far away". The gist is that officials in the outlying provinces can ignore Imperial decree because it would take more effort than it's worth for the Emperor to punish them. But when enough of the empire is ignoring the Emperor, well...it's not really an empire anymore.
Now obviously, if your magical big boom is core to the plot, then by all means keep it. But if it's just a plot device to explain a broken empire, there's really no need. Empires tend to shatter under their own weight.
On a different note: where's the old Imperial core, on A, B or C? And keep in mind that continent-sized landmasses should be geographically diverse, not "one big plain" or "all hills". If the landmasses are considerably smaller than continents, you can have that but even then small landmasses often have remarkably geographic diversity.