This is a pretty straightforward story driven quest. In a nutshell...
After you complete the Meeting with Corvus mission, you will need to speak with Tassyn. This will continue the story line and unlock Strongholds for your participation. Proceed to Faye, and talk with her. This should unlock the Tombs of the Legionnaires mission. From here, proceed into Freeplay and search for the four Legionnaires' tombs. The tombs are:
Tomb of Artinia
Tomb of Cariff
Tomb of Gawnes
Tomb of Yvenia
Every tomb has a different set of Challenges to complete before you can access that particular tomb. The Challenges will be a variety of task such completing objectives in Freeplay, doing Strongholds, and defeating enemies.
Once you've completed the separate Challenges, return to each tomb to enter it and collect the rewards contained therein. After unlocking all four tombs and collecting the rewards, you will be able to unlock the Tomb of General Tarsis. Return to Fort Tarsis, talk with Faye to start the mission. The mission is a decent challenge, complete with numerous enemies and a few puzzles. Once you've gotten through the Tomb, escaped again, and returned to your Strider, the achievement will unlock.
Good Luck!!