Good creatures are called the AcE
Bad creatures and arch-enemies of the AcE are called the AxE

AcE live and make a living by using the life force of other living entities, such as animals and plants
The animals and plants whose life force is used by the Ace are not killed
Obtaining the life force of another entity grants an AcE their unique abilities for temporary use
AcE will destroy other AcE if they try to use that AcE's life force
AcE who destroy another AcE for their life force become one of the AxE (at least eventually - may take from 5-10 kills, and be a gradual transformation)
On the other hand, an AcE who is given the life force of another AcE by the will of that AcE becomes more powerful without being tainted
With enough AcE life forces willingly given to them, a pure AcE becomes an i/oAcE - one with supreme abilities and power to overcome the AxE

At a crucial half-point in the game, the main AcE learns from the leader of the AxE the horrifying truth that AxE were once AcE
The leader of the AxE was a hero of the only civil war in the history of the AcE, tainted by the lives of other AcE he slaughtered
The leader of the AxE is really an i/oAxE, because to have victory against his enemies, the rest of his comrades gave him their life force
The history and story of this war is hardly known to any but an elder or two who keep their mouths shut about it to most

A fellow AcE slowly becomes an AxE over the course of the game - a recurring boss, if you will
Perhaps he tries to or does become another i/oAxE? (maybe the original i/oAxE gives this special AxE his life force right before he is defeated?)

The first half of the game is spent fighting AxE bosses who guard keys to the keep of the i/oAxE
Once he is reached, the aforementioned half-point comes (with or without a fight - if with a fight, maybe it is here that the ex-comrade becomes the new
     i/oAxe, and before the fight is when the truth is told?)
The second half of the game is spent fighting AxE bosses who are holding other AcE captive; most or all of these AcE willingly lend you their life force
     after the boss is defeated, thus giving you new abilities
By the end of the game, you become the only i/oAcE to have ever existed, and concquer the last i/oAxE
Perhaps the above is acheived by an AcE who has been following you, who finally decides to grant you their life force?