What the achievement asks
Turn in a total of 30 Restored Obsidian Keys to the figures around the Obsidian Citadel. Ten points, account-wide.
The arithmetic
Each key is 30 Key Fragments and 3 Key Framings, both dropped by the mobs inside the Obsidian Citadel in The Waking Shores. Thirty keys is therefore 900 fragments and 90 framings, which makes this the long grind of its branch.
There is no daily cap and no weekly cap. It is purely a question of hours in the Citadel.
Where to farm
The two basement spots are the ones people return to. They are enclosed, the mobs come to you, and there is no travel between pulls.
The spawn rate responds to the number of separate groups at a spot rather than the number of players. Standing at one basement spot as two parties of four rather than one party of eight makes mobs arrive noticeably faster, and it is the difference between an evening and a couple of hours. Groups form for this in the group finder under names like "Key Basement".
The mistake that costs whole runs
A completed key disappears from your bags if you leave the Obsidian Citadel. Fragments and framings are safe to carry; assembled keys are not.
Turn every key in the moment it completes. Not before you log off, not before you hearth, not before you fly out to check a rare. People lose entire evenings to this one behaviour, and there is no way to recover the keys.
Spread them across all four
There are four people who accept keys: Wrathion, Sabellian, Forgemaster Bazentus and Igys the Believer, who stands with them at the Citadel.
Do not dump all thirty on one of them. Each carries an event bar, and filling all four is Obsidian Champion in the same branch. Handing one key to each also closes I'm Playing All Sides, and the keys given to Wrathion and Sabellian raise the friendships behind Loyalty to the Prince and The Obsidian Bloodline.
Five achievements come out of the same 900 fragments if you spend the keys thoughtfully, and only one of them if you do not.
A mount, while you are here
Caches from Igys the Believer have a chance to drop the Worldbreaker Membership, which is what gates the Slumbering Worldsnail Shell mount. It has nothing to do with this achievement, but you will be handing him keys for hours either way.
Where it fits
One of eleven under Flight Club, a piece of A World Awoken.