Basically, the Great Curse was the dying last spiteful action of dying Primordials, before they become the giant corpse-tombs known as neverborns. In many ways, it lets the exalted become More of themselves.. but also tends to cause them to occasionally go psuedo insane. Kasumi, back when she was trying to make Jacob tell her how to improve her life? That was great curse.
Its the biggest secret in the setting, that only two people knew about it. Due to ones nature, she can't tell anyone until its the right time, while the others been trying and failing to cure it. Autocthon is the third person to discover it, and the one with the best chance. Revealing the great curse to the world is one of the greatest ways to cause mass chaos, as it makes actions taken in both the past and present incredibly suss.
Did they really do that because they wanted to, or because they were forced by the great curse etc etc, so the exalted will be massively gripped by an existential crisis.
Fixing it is basically impossible, with the only known way is for the neverborn to invert their exaltions essence. Even if someone manages to cure it, the exaltions will be weaker afterwards, unable to take advantage of some of the power that such things grant.
The in game representation is basically limit break. When one of the Exalted goes into Limit Break, they suffer from the temp insanity. Once they recover however, they realise how 'damn good' they feel, cleansed off stress and stuff. They also get all their willpower refreshed, allowing them to use their best powers again.
