on some islands you will see houses with special characters to unlock that cost diamonds. try to recruit them to your base because they are important and each one serves a specific purpose.
in one of the big islands you will see a big dead tree with two (also dead) trees on each side. when you pass over the middle, the cost of it will be shown with the "special character" to unlock it.
locate that guy in your base, carry him to the dead tree and pay it to unlock.
*** Spoiler - click to reveal ***The trees will become alive, and a giant golden apple will appear on the big tree. Put a coin on it and the apple will drop to the grass floor. Picking it will make something special happen and you will be granted a special shield, which will have the power of petrify anything forward your mount (also your own troops and even your companion king if you are playing on split screen or online. It is useful because it recharges faster than other powers, Press LB again to keep freezing everyone on your sight.
If you are using this power it this will unlock naturally, when 20 subjects get petrified it.
I am aware that some descriptions are kind of "vague" but this game is really enjoyable and I only want you to guide a bit about what you need to do, without spoiling things a lot, since the first time you do this is a "WOW" moment that you will experience once.