Fury activates when the colossus is at max mana but he keeps using "gather magic" for 10 more seconds. It takes all his mana and prevents him from regaining it in exchange for giving him the power to unleash ultimate destruction.
It has to be a bonus... a nice bonus.
We already got ideas about the Infernal and Stygian colossus:
Infernal creates like 5 volcanos in a circle around him. They spit out fireballs that fall on people's heads and turn into mini infernals.
Stygian kills everyone around him and turns them into zombies.
Your turn to post ideas!