When an upgrade for a Heroes Special Skill is available, it is indicated by a green 'S' next to the Special Skill icon of the Hero
Increasing a Heroes Special Skill requires Skillbooks.
There are 5 different rarities of Skillbooks:
- Uncommon
- Common
- Epic
- Rare
- Legendary
The higher the Special Skill levels (to a maximum of 10) the rarer the Skillbooks are to obtain.