What the achievement asks
Complete two Zandalar storylines: The Fallen Chieftain and Justice For The Fallen. Ten points, account-wide, Horde only.
Two chapters, one story
These are the two halves of Vol'jin's aftermath in Zandalar, and they run in order: the first deals with the fallen chieftain, the second with what is owed for it.
Neither is part of a zone's main quest chain, which is why they can be missed entirely by a character that quests right through Zandalar without looking sideways.
What has to be done first
Welcome to Zandalar opens the continent, and the zone stories come before this. Once those are behind you, the two chapters here are short.
The thread continues in The Shadow Hunter, which follows Vol'jin further. That one is earned per character rather than account-wide, so it is worth running on the same Horde character you intend to keep.