League Quests are an essential part of the game. The purpose of these Quests is to reward players for their teamwork and collective strategy.
You can earn up to four Reward Chests during a single Quest: Silver, Golden, Platinum, and Adamantine Chests.
Chests are stored in the "My Items" menu, and are valid for 72 hours after you receive them. After this time, the Chest expires and you will not be able to open it anymore. Make sure to open your Chests as soon as you get them so you don’t miss out.
Unlike individual Quests and Tournaments, League Reward Chests are given out not when the Quest ends, but at the moment the League fulfills the requirements for each Chest.
This means you can open the Reward Chests while the Quest is still in progress. This can be confusing, because when the Mission ends, the list of unlocked Reward Chests is displayed again, even when a player has already opened all of their Chests.
If you can't find an earned Reward Chest in the "My Items" menu, you almost certainly opened it earlier.