system
Given the following post, respond as if you're commenting on Reddit:
human
[League of Legends] Why was the league created in the first place? By who and when?
gpt
This is a little tricky to answer in-universe because a lot of the lore has recently been removed or rewritten, but before that happened:

The continent of Valoran suffered from constant war, including magical warfare that was literally destroying the land. To keep everything from being destroyed, powerful magicians called Summoners banded together and established the League of Legends as a more peaceful means to resolve political disputes. Instead of fighting with armies, Summoners would summon a team of champions to fight instead. These battles quickly replaced traditional war and also became the most popular pasttime in Valoran. 

Most champions joined the League voluntarily, either to represent their city-state, or for fame, glory etc. But a few who are considered too dangerous (such as Fiddlesticks) are imprisoned in the Institute of War (league headquarters) by the Summoners and are only allowed out to fight.