What the achievement asks
Reach maximum friendship with Sabellian at the Obsidian Throne. Ten points, account-wide, and it grants the title Paragon of the Obsidian Brood.
Sabellian is Wrathion's opposite number at the same location, and the two achievements are the same work aimed at different dragons.
Where the friendship comes from
Obsidian Keys. Each Restored Obsidian Key handed to Sabellian raises his friendship and counts towards Obsidian Keymaster. A key is 30 Key Fragments and 3 Key Framings, both from the mobs inside the Obsidian Citadel.
The weekly quest. Allegiance to One asks you to declare for Wrathion or Sabellian each week. Declaring for Sabellian points that week's work at him.
Alternate weeks. Loyalty to the Prince wants the same maximum friendship from Wrathion and sits in the same branch, so the two are not a choice between rivals: they are two halves of one grind, and picking the same dragon every week doubles the total time.
Where to farm the fragments
The basement spots are the dense ones, with mobs walking into a confined space rather than needing to be chased across the Citadel.
Spawn rate responds to the number of separate groups present rather than the number of players, so eight people split into two parties of four at one spot pull far more than the same eight in a single party.
A completed key disappears from your bags the moment you leave the Obsidian Citadel. Fragments and framings travel fine. Hand keys in as they finish.
Why to start this early
Like its twin, this reputation has no renown track, no catch-up and no weekly bundle behind it. It advances only while you are standing in the Citadel killing things, which is why it is so often the last piece of the branch left when everything else has capped.
Treat the Citadel as the thing you go to whenever you are waiting for a timed event elsewhere on the Isles, and it fills itself in while you wait.
Where it fits
One of twelve under Oh My God, They Were Clutchmates, a piece of A World Awoken.