From the introduction:
For years, periodically, the Lich-Lord has been sending waves and waves of undead into civilized lands. Every century or so, he is beat back, only to come back with a new army of undead every century or so. So it has been [since time immemorial].
However, despite the setbacks that this situation has caused, civilization has become more and more sophisticated over time, and several great civilizations have clawed their way into the light. And this time, they're taking the fight back to the Lich-Lord.