The Hero tree was a later addition to Game of War and allows you to unlock a variety of additional abilities. You now need to unlock Monster Masteries in order to attack monsters on the world map. There are also a number of very useful abilities such as increased troop training, health and defense.
The Hero tree also added some really high resource requirements and very long timers. You need to spend a significant investment in this tree in order to unlock all of the Monster Masteries.
A breakdown of the abilities in the order they are unlocked is show below:-