As a creative writer myself, I must say this is a really fascinating story, and I can't wait for Chapter 5.
There are only a couple things I have to say to "critque" it though. The Voidborn that Fiora and Lux first encountered share their name with Kassadin's title, the Void Walker.
Also, I'm not the biggest fan of Swain and LeBlanc being possessed or whatever they are, as I feel it would spice things up knowing that each Champion is still themselves as the darkness of the Void closes in around them and how each of them deal with it, but that's just my personal preference.
Then I'm not entirely fond of how Caitlyn was introduced. I thought she was the type to track down criminals herself, as oppose to leading a small army from a meeting room or something. Also, her attire doesn't seem to match what she "usually" wears.
Apart from those minor things, excellent work. Little to no spelling errors even in such a large amount of text like this; that's very impressive. I also like how you portray the characters, and it's an interesting thing on what you did with Fiora. As said, can't wait for the next chapters, and hopefully the Void-based guys (Cho'Gath, Kog'Maw, Kassadin, and Malzahar) will play a significant role (was kind of hoping for them to show up in the first few chapters).
This actually kind of entices me to create my own LoL fanfic, though I already made 40 pages worth of mini-fanfics when creating my own Champions XD. Will have to post them sometime...
Anyways, again, awesome story and looking forward for more.