Original reading guide by The Rune, based on the linked publications. Suggested reading order, not an official dated timeline.
Not another ordinary country
The Void is described as an unknowable absence beyond the material world. Its manifestations threaten Runeterra, but it should not be read as simply another nation on the map. The regional account distinguishes the Watchers from the creatures created to bring destruction. A champion's connection to the Void does not automatically make them one of its willing servants.
The world meets the impossible
Different stories describe breaches, corruption and the attempts to survive them. Icathia is an important historical entry point, while Kassadin and Kai'Sa bring the consequences closer to individual lives. These perspectives explain what contact costs; they do not provide a complete or universally agreed taxonomy of everything beyond reality.
Read survivors and powers separately
Start with Kassadin and Kai'Sa to understand survival and resistance, then read Bel'Veth as a distinct power with her own ambitions. Do not flatten those aims into the goals of every other Void-associated being. Older profiles vary in depth and continuity; publication labels and sources matter particularly when a short origin story is being used to explain the whole threat.