It never occurred to me until recently, but we have a lot of information about the Watchers failed attempt to invade Runeterra.
This post is meant to gather that information, recap what happened in rough chronological order and serve as a tl;dr if you're not interested in searching through different bios to find the clues. So if I missed something (which I most likely have) please let me know in the comments and I will edit the post.
That being said, let's get started.
The Void is a realm beyond the martial plane. It is a cold and empty place. The only entities existing in it are drifting through the nothingness, not even fully aware of their own sentience. These entities are called "The Watchers", although they don't live up to that name yet.
Then, something changed.
It is not know whether our reality came into existence or it's existence just then started to collide with the Void. What matters is that the Watchers became self-aware and decided that it could not be tolerated. They started watching our reality and searched for a way to invade this realm and silence it's existence.
Eventually, they saw a chance when human minds started to reach out to them.
##Lissandra, Serylda and Avarosa
One of those minds was that of Lissandra. Blinded by a primal god, in times long past, she started to walk in dreams and, willingly or unwillingly, communicate with the Watchers. The Void was already known as a threat, but Lissandra saw more than that in it. Enticed by the infinity promised by the Void, she struck a deal with the Watchers. Lissandra and her sisters, Serylda and Avarosa, were granted near-immortality in exchange for preparing the world for the invasion of the Watchers.
However, Avarosa and Serylda didn't share her point of view (_insert blind joke here_). Lissandra tried to stall the Watchers for more time while soothing her sisters concers, but the Watchers did not care. Avarosa and Serylda gathered their allies to fight the Watchers, while Lissandra started to have doubts whether or not it was a mistake to help the Watchers. Before this inner conflict could be resolved, the Watchers invaded through a rift in the Howling Abyss.
##The Howling Abyss
Before Lissandra was blinded, Avarosa was deafened and Serylda lost her voice, the three sisters approached Ornn. They asked for his help in the fight against the Void. Ornn didn't want to get involved in that war, but he was willing to help by digging a trench that could be used to trap the Watchers.
We don't know how the rift to the Void formed precisely at the bottom of the Howling Abyss, but it was most likely put in that place by Lissandra.
Before the invasion of the Watchers, the Yetis were a noble magical race who ruled over the mountains of the Freljord. In the current time they are known to be savage beasts, yet they were allies to the humans in the fight against Void. Their importance to this story comes from their ability to create True Ice, ice that never melts.
The Watchers entered Runeterra. Yetis, Avarosa, Serylda, and all of their allies have gathered to fight them, including Gnar (but he didn't really know what was happening). Lissandra was faced with the decision whether to allow the Void to consume all or to betray and sacrifice her sisters and all their allies to prevent it.
She chose the latter.
We don't know exactly how she did it (most likely by stealing the magic of the Yetis), but Lissandra created a massive layer of True Ice, freezing everything and everyone inside the Howling Abyss for eternity.
Or so she hoped...
It turns out that True Ice was not enough to stop the Watchers, only to stall them while they slowly corrupt the ice into Dark Ice. Lissandra and her followers founded the "Frostguard" and started rewriting history to cover up any traces of what she's done. (I think this is a hint from Riot that stories like "[The Lost Tales of Ornn](https://universe.leagueoflegends.com/en_GB/story/tales-of-ornn/)" might not be 100% accurate.) Since True Ice is able to stall the Watchers, the Frostguard are gathering as much as they can find to delay the inevitable.
The Yetis who have lost their magic turned into savage beasts, while those who didn't were hunted by the Frostguard to create more True Ice.
Also, Gnar managed to break out of the ice somehow and is currently exploring modern Runeterra.
The rift in the Howling Abyss was shut. The Watchers who stayed in the Void and weren't part of the invasion needed a new way to attack and learn about this reality. They started to shape the matter of our world to create the Voidborn, constructs designed to be the eyes and ears of the Watchers.
Vel'Koz is the oldest Voidborn alive and is relaying all of his knowledge to the Watchers to this day. More modern Voidborn have different purposes. Rek'Sai, for example, is creating another rift to the Void, although it is unknown whether that allows Watchers to enter Runeterra or just more Voidborn constructs.
[The Eye in the Abyss](https://universe.leagueoflegends.com/en_GB/story/the-eye-in-the-abyss/)
[The Lost Tales of Ornn](https://universe.leagueoflegends.com/en_GB/story/tales-of-ornn/)
[Nunu & Willump biography](https://universe.leagueoflegends.com/en_GB/story/champion/nunu/)