What the achievement asks
Finish the Rebel Dragonkin storyline. Fifteen points, account-wide, and it grants the title The Reconciler.
The three gates in front of it
This quest does not appear in Valdrakken until two other things are done, which is why people search the city and find nothing.
First the Thaldraszus campaign, then the Misty Vale storyline, which is one of the eight in Sojourner of Thaldraszus. Only once Misty Vale is closed does the Rebel Resurgence chain that this achievement wants become available.
None of that is wasted work. Misty Vale is needed anyway for Loremaster of the Dragon Isles, so doing Thaldraszus properly opens this on its own.
What you are doing
The chain runs from Cause Without a Rebel to Reconcilable Dissidents, which is the quest that awards both the achievement and the title.
It is story content: solo, no farming, no randomness, no group. The whole run is an evening at most, and much of the length is dialogue rather than travel.
The tracker shows the criterion as Completed Reconciable Dissidents., with a letter missing. That typo is Blizzard's own text rather than a mistake on our side, and it is the same quest as the one linked above.
If the title arrives and the achievement does not
When this storyline first went live, players reported receiving the title at the end without the achievement appearing. Their total achievement points went up by fifteen all the same, which meant it had been awarded and simply was not displaying.
That has long since been fixed, but if you see the title and no popup, check your points before assuming anything went wrong.
Where it fits
One of six under Dragon Quests, a piece of A World Awoken.