What the achievement asks
All four covenant campaigns finished. Ten points, account-wide, both factions.
Nine chapters each, gated by Renown, and the whole of Shadowlands' story sits inside them.
You do not need one character to do all four
This is account-wide, and it counts a campaign wherever you completed it. A Kyrian paladin, a Necrolord death knight and a Venthyr warlock between them satisfy three quarters of it.
That matters, because most people who played Shadowlands already have one or two of these done on characters they have not touched since. Check before you commit to anything: you may be closer than you think.
For the ones you are missing, switching covenant on a single character works too, and switching now costs one visit to Oribos.
What each campaign costs
The chapters unlock at Renown thresholds rather than on a timer, and Renown catches up quickly for a covenant you join late. In practice a campaign is an evening or two of questing rather than the months it took in 2021.
The Venthyr snag
A specific and well-documented one: the Venthyr campaign can sit at 8 of 9 with the Sinfall chapter refusing to appear.
The fix is not in Revendreth. Push the Korthia storyline until the Medallion of Wrath quest becomes available and complete it, and the last chapter unlocks. People sat stuck on this for days before finding it.
Where it fits
One of the sixteen pieces of Back from the Beyond, and it overlaps almost entirely with Re-Re-Re-Renowned: the Renown you push for one is what unlocks the chapters of the other.