For this achievement you must collect all of the shards in the game, then bring them to the temple in the forbidden oasis and enter all of the doors. There are three sections of the temple, each with three doors to open. The first door to each section needs 6 shards, second door 12 shards, last door 18 (credit goes to Sa1rcor). Once you unlock all these doors, you go back to the beginning of the temple and enter the main door, and the achievement will pop. To find shards, you have to find an ocularum (a skull icon on your map) then use it to "look" for the shards, then they'll appear on your map. To unlock the forbidden oasis, find a shard in the Hinterlands and bring it to your researcher at Haven (or Skyhold if you're starting late) then do the war operation to unlock it.
(Credit to these maps goes to reddit)
http://www.reddit.com/r/dragonage/comments/2nfgx7/spoilers_d...Just click on the "collections" link for each location to see where the skulls are, then follow them on your game map. If you would prefer a video guide, credit goes to segmentnext
http://segmentnext.com/2014/11/24/dragon-age-inquisition-ocu...