This is story related and can not be missed.
After you have completed the quests to gain support of the Elves, Dwarfs, Mages and the Arl of Redcliffe ask the Arl to convene The Landesmeet.
This will start up the major quests in Denerim. Once you have completed these travel to the Royal Place and confront Loghain. Once this is done the achievement is yours.
One note if you plan to reload saves in order to get all the different endings in one playthrough your best bet is to save before travelling to the Palace. This save will also be useful if you find you have missed anything for the Traveller achievement as all the missable area's and buildings can still be reached before the Landsmeet.