(Upon entering the beach of the Labyrinth Shard challenge, don't forget to head left to get the Lorestone before double-backing and going right to enter the burial mound).
So this burial mound/labyrinth has a few tricks to it, the most important one being...
In case you get lost, here's a map to help. Image credit to GameFAQs and GameFAQs user burqawitz:
To go through the tomb and pick up the three Lorestones held within, here's the sequence to follow:
Left, Left, Left into dead-end with Lorestone 1, Back, Right (you'll be entering this room on the "left", so make sure you don't accidentally go back again), Left (center of room has Lorestone 2), Right, Right, Left, Right, Right, Left to dead-end with Lorestone 3, Back, Back, Back, Right (you'll be entering this room on the "left", so make sure you don't accidentally go back again), Enter the broken wall, Right. Enter the red doorway in this room to get the achievement.
To go directly through the tomb and avoid the above problem (though you'll miss Lorestones), here's the sequence:
Left, Right, Left (center of room has Lorestone 2), Right, Right, Right, Right, Enter the broken wall, Right. Enter the red doorway in this room to get the achievement.
A few more random items:
*** Spoiler - click to reveal ***See that seal on the ground with the tree a few rooms into the labyrinth? The tree represents the labyrinth, with the trunk as the entrance. You can follow this map straight to the end (although you'd miss a Lorestone or two on the way)
Certain rooms will have torches in the center. Lighting these is helpful, as if you ever enter a room with a lit torch, you know you've been there already and are caught in a path loop.
See the shields hanging on the wall in the center of the room? If you ever see two of the same shield in a row, you're in a path loop.