Devastation

The Devastation was an almost world-wide environmental disaster caused by a group called the Circle. They had discovered magic and determined to keep its secret from all others, using their new-found powers to conquer the known world. They successfully built an empire, but no empire ever lasts.
When forces allied against them so began the first great war - a war they began to lose. They had planned for this eventuality and switched to their alternative plan of unleashing magic and disease onto the world through a series of bombs planted all over the region. When detonated, the blasts killed thousands immediately, but the released pathogen accounted for virtually everyone else.
The world was almost entirely laid waste - even more so than the Circle had predicted. They had retreated to protected underground cavern systems along with hand-chosen people and various lifeforms, ready to repopulate the world - a world they would then have complete control over.