Dec 12, 2018, 02:0912/12/18
Hey Borse
Welcome to the forums, that key is for attacking clan bosses.
You do need to be in a clan to use it and once used, it takes 6 hours to get another one, or you can buy one for 100 gems.
Though its worth noting that the key should be used as soon as possible to allow the timer to start to get another one as you can only hold 1 at a time.Cheers