What the achievement asks
Reach maximum friendship with Wrathion at the Obsidian Throne. Ten points, account-wide, and it grants the title Agent of the Black Prince.
This is one of the twelve reputations of its branch, and one of the ones that sits outside the renown system. There is no weekly lump, no catch-up and no story quest that hands you three ranks. It moves at exactly the speed you farm it.
Where the friendship comes from
Obsidian Keys. Each Restored Obsidian Key handed to Wrathion gives friendship as well as counting for Obsidian Keymaster. A key is 30 Key Fragments and 3 Key Framings from the mobs inside the Obsidian Citadel.
The weekly quest. Allegiance to One asks you to pick Wrathion or Sabellian for the week. Choosing Wrathion sends that week's turn-ins his way.
Picking a side is a weekly choice, not a permanent one. You can raise both to maximum by alternating, and you need to: The Obsidian Bloodline wants the same thing from Sabellian and both sit in the same branch.
Where to farm the fragments
The two basement spots are the ones players keep coming back to: enclosed, dense, and the mobs walk to you rather than the other way round.
Spawn rate at these spots scales with how many separate groups are standing there, not with how many people. The trick players use is to stand at one spot and form two parties of four rather than one party of eight. Mobs then come noticeably faster for the same number of players, which turns an evening into an hour.
Keep an eye on your bags while you do it. A finished key vanishes if you leave the Obsidian Citadel, so hand each one in as it completes rather than saving them up for the flight home.
Why to start this early
These minor reputations have no shortcut. Every other part of the meta has a weekly quest, a renown track or a catch-up mechanic behind it. This one has an hourly rate and a total, and it is regularly the last thing standing between players and the branch, long after the renown factions have capped themselves.
If you are working through the expansion's meta, start the Obsidian Citadel in your first week and dip into it whenever an event you are waiting for is not up.
Where it fits
One of twelve under Oh My God, They Were Clutchmates, a piece of A World Awoken.