Hero promoting was introduced in the "Manifesto" Expansion on July 10, 2013. Promoting heroes is used for inreasing the "rank" (stars) of a hero while keeping skills. White heroes cannot be promoted. However, you can promote green+ heroes. Gold crystals were also implemented.
|Hero Rank||Requirements for Promotion|
|Level||Yellow Crystal||Green Crystal||Blue Crystal||Purple Crystal||Golden Crystal|
|Green -> Blue||50||200||100||20||0||0|
|Blue -> Purple||65||0||200||50||10||0|
|Purple -> Gold||80||0||0||200||100||20|
The golden hero was introduced in this expansion also. A golden hero gets 8 primary and 4 secondary stats.