Elysian keys are limited time in-game currencies used to unlock Elysian Caches. Elysian Keys are earned by completing a specific daily challenge. Each freelancer can only earn 1 Elysian key every day during a period of 60 days.
At the end of each stronghold, 4 Elysian caches appear which the player can unlock using these keys. All four can be opened by a single player at once if enough keys are collected, or each freelancer in the squad can open 1 chest.
At the end of the event, the Elysian Keys disappear, so it is best to open all the keys, even when all the cosmetics are unlocked, especially to help other freelancers to get these cosmetics.