So how do story events work?
Have you ever thought "I need more chaos in my life," or "these independents just aren't causing enough trouble"?
Well, story events are a good way to accomplish both!
Basically, they're high-risk, high reward events/chains of events that are unleashed on your world. A lot of them are tied to thrones - for instance, when someone claims the throne of beasts, the entire world randomly gets groups of werewolves and giant dogs attacking it. Or the throne of death, which causes undead to attack everyone.
"But why would I want random indies causing chaos everywhere?" Well, first, it's fun, but secondly, if you're the one who actually set off the chain (or if you can capture the throne quickly enough) you'll get a bunch of free troops. I forget what the beast throne gives you (probably a bunch of werewolves), but the bones throne gives you some undead commanders and several hundred chaff undead troops.
Other events include unleashing a daemonic portal which will then throw demons at everything until someone manages to make it stop (if I remember correctly, if your mages discover a heavily runed, locked door - leave it alone), and getting a bunch of greater otherness' by sacrificing some population (was related to some fishing village who your mages discover is worshipping Cthulhu.)