Suggestions on how to change it?
Rewrite it completely.
A question: is it that you want
orcs to be an Always Evil Kill On Sight monster, or just that you want there to to be an AEKOS monster in your game. If it's the latter, why not use fiends? Maybe in "an age before history," some relatively low-level fiends (which don't have to have anything to do with Blood War politics and have no connection to more powerful fiends, unless you want there to be one) got loose on the Prime and have been reproducing. It doesn't have to be through making babies the old fashioned way, if you don't want that. It could be spontaneous generation, through fission or budding, implanting larvae into victims, stealing and transforming souls, or any number of other things.
Or it could be that, in that age before history, certain places or substances became corrupted (fiendish or Far Realms touch, or something else entirely), and beings who are exposed to the place or substance become corrupted as well, transforming into AEKOS beings, and there's no known cure beyond certain very high-level magics.
But if you still want it to be orcs, why do they need to be AEKOS? They can be "bad guys" in other ways. Humans farm and ranch. Orcs hunt. Orcs don't care about or don't understand fences and damage farms and ranches in the process of hunting; maybe they even hunt human-raised animals when wild game is scarce. This leads to humans wanting revenge, either through wars, raids, bounties, or just propaganda. Orcs retaliate against
that. And so on.
Switching back to your idea for a moment. Even if you, the DM, have decided that the true identity of the Unknown Evil is, in fact, unknown, other people in the world aren't going to be satisfied by that. They're going to decide that some evil god or demon in the pantheon is responsible--or some not-evil god did it, as a punishment for something humans did at the time. Or decide that it was actually wizards or elves.