What the achievement asks
Complete the Old Hatreds storyline in the Ohn'ahran Plains. Five points.
Blizzard ties this one to the character, not the account. Doing it on an alt does not close it for your main, so pick the character before you start rather than after.
Where it starts
The chain begins with A Debt That Must Be Paid in the Ohn'ahran Plains. Everything after it is handed to you in order, so that one quest name is all you need to find.
It is story content: solo, no group, nothing farmed, nothing random. An evening at most, and much of it is dialogue.
What you get for it
The questline rewards the toy Display of Strength.
If you run it on a tauren or a Highmountain tauren, you also receive the Ancestral Bloodhoof Totem and Ancestor's Might on top of the toy. Nothing else in the chain changes, and no other race gets them.
That matters more than it looks, because the achievement does not carry between characters. If the character chasing the meta happens to be a tauren, the extra rewards come free. If it is not, do not run it twice for them: the toy is the same either way and the achievement only counts where you need it.
Where it fits
A piece of A World Awoken, the Dragonflight meta, which rewards Taivan.
The Ohn'ahran Plains hold several other things worth clearing on the same trip: the Grand Hunt locations of The Disgruntled Hunter, the zone's races, and the storylines of Sojourner of the Ohn'ahran Plains that Loremaster of the Dragon Isles wants.