Chaos is the central enemy of your gun-toting wizard. By the time the game starts, it has already shattered the world and won, so your only choice is to return to The Tower and rewind time, hoping that eventually you'll figure out a way to stop it.
In terms of mechanics, Chaos is set to destroy the world within five minutes of you starting an expedition. The counter can be delayed by destroying Rifts you come across (an Ambush Rift delays Doomsday by 30 seconds, a Dormant Rift by 3 minutes, while a lurking Eye of Chaos miniboss by another three). Once counter hits zero, the world ends and Chaos starts appearing across the map, destroying tiles and spawning high level enemies. While this does yield a ton of Arcana, it's also very dangerous and routinely spawns enemies above your character's level.
Surviving five minutes in a doomed world does unlock an achievement though.