My Lands: Black Gem Hunting

Like we have the archer->airborne->cavalry->infantry->archer thing, there could be similar thing for races

Knight can have bonuses against demons, who have bonuses against elves, who have against Dark Elves, & they'll have bonus against Knights

So Knight->Demon->Elves->Dark Elves->Humans same way it is with units.

If you want a backstory, It could be something like:
>The order of Knights, which now control human society, were created to cleanse the plague that was Demons.
>Demons are the Elve's natural enemy and have evolved to hunt them.
>The first dark elves were exiled for Xenophobic behavior towards humans. The hatred has brewed in them since then. Blaming the human race for their exile, they kill humans with ruthless rage and madness.
>Elves, proud creatures as they are, despise seeing what they might become when twisted by hatred. They constantly try to obliterate any signs of this possibility, taking the impossible task of destroying all Dark Elves.

the idea is somehow similar to what we already have with faction. You get it when attacking players of enemy faction.

This calls for more tactics.

How does the enemy faction thing work again?

You get it when attacking players of enemy faction.

If Light Faction attacks Dark Faction, Light gets bonus against Dark?
But if Light is enemy of Dark, why doesn't Dark get bonus?

I'm suggesting that some races are always weak against another race.

Eg: If I(Elf) want to take revenge on a Demon, It will be better to ask my faction ally, Knight, to attack, as Demon will have bonus against Elf even while defending.
Then the faction thing, which is basically an alliance between races, also makes more sense: Elves need Knights to deal with Demons, and Knights need help with Dark Elves. (Same for Dark faction)