Quote Originally Posted by Randombob731 View Post
? Explain please
Sometime down the road, we will have the option of trading golds and higher for rainbow crystals (RC). You can trade RC for different units that will be made available. The higher the rarity, the more crystals you will get when you exchange the unit. Also, shrine units will net a higher yield than event units.

This feature will not be available for a long time. So you will have time to reconsider what units you want to trade, and which units you want to combine.

EDIT: And Soara beat me to it