For this achievement, you have to find all of the shards in every area of the game. To do this, you will have to find an ocularum (skull on the map) and guide it to points of light to make the shards show up on your map. Then you will have to go walk to each one and collect them individually.
A good tip for making them easier to find once you've marked them on an ocularum is to place a waypoint on the next shard you are going to, as it will emit a blue pillar of light into the air above the shard.
Some shards require some tricky (if not seemingly impossible at first glance) jumping. For these, I recommend switching to Cassandra, because for some reason it seems like she jumps higher than the Inquisitor does, not paricularly sure why. Especially when getting the shard in the middle of Lady Shayna's Valley in the Hinterlands (you'll see what I mean).
Once you've acquired all of the shards, travel to the Forbidden Oasis (far left on the world map) and travel to the northernmost part of the map, going through some tunnels from the Intrinsic Pools region to find the Solasan Temple. Inside you will use the shards you've acquired as keys to go through various doors to fight waves of enemies and collect loot and stat bonuses (very useful btw). Once you've unlocked the final door the achievement will pop.
PS: If you can't figure out which areas you have shards left in, hover over them in the world map and hit LB to toggle through your quests to see if you have a "Shards in ___" open there still. If you do, there are some shards left to be found. There are 104 total shards in the game.