Thats awakening, which we don't have yet on Nutaku, thought we may get it soon as JP got it after this event. In order to awaken a unit you need to max level and 100% affect a unit above gold rarity. After that you need to farm awakening orbs from special dailies, buy an awakening fairy from the trading post for 100 demon crystals, then finally pay a gold price in the 100-300k range depending on rarity. You also need 3 CC'ed silvers or gold units to awaken one.
It's a very time consuming process, but after all that you can awaken a unit, which raises their lvl cap and unlocks a special passive for that unit.