Yo get the the tomb you need to do the tenth forge weapon thing that gives you the sword
Once you do that you will get a quest which gives you a shotgun,emblem, and you see saint-14’s tomb
English
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Excuse my typing*^
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The takeaway from my post is the keyhole. It's open. I've completed all those quests and been in there. But if you load up a mission and go to the future? You'll be able to see inside the tomb by jumping up to the door. It's a good nod towards Saint-14 and his legacy.