Chaos's emotions are not the same as humans', or even as most gods'. They predate and are the ultimate source of - well, everything - so in a certain sense they have all possible emotions. But in a different, equally true sense, they have none. All of the seeds of potentiality are contained within them, and they pick which to nurture at any given moment in spacetime based on whims not to be understood by mortals.
All of that said, it is not an unreasonable gloss to say that all of what follows happens because it amused them.
It starts, like many other situations that amuse Chaos, with a certain Prince of a certain Underworld (and this time it is much less the Prince of the Underworld than it typically is) being offered the choice to enter their realm. They do not know whether the Prince will accept this choice this time, but that is just the same as saying that there are some worlds where he does accept the choice and some worlds where he doesn't, and Chaos is equally interested in both.
This, then, is the story of one such time when he does accept the choice, and of what happens after.