Mar 14, 2019, 00:4103/14/19
Hey Dave!
The key is for defeating the clan boss! Once you've joined a clan you can use your keys to fight the boss, which refreshes every 6 hours.
For more of an in-depth talk about the Clan Boss, you can check out Marius' post on the forums!
https://plarium.com/forum/en/raid-shadow-legends/673_guides-and-tutorials/117636_guide-clan-boss/
Hope this helps!