So shortly after buying the legendary King dragon and using up 100 relics which lets not forget costs 1000 gems to activate I find that you can get a token for him by simply defeating him in the campaign!

Really if something costs this much yet is available later on in game for nothing there should be a warning attached to it. By the time you have the hundred relics needed to activate him you are nearly at him.

Btw it is possible that the late game balance is a little off as whilst at the start you might struggle on a level later on it is just a case of leaving your heroes to it and tapping on the chests when they are done.