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Once you have one of these, you want to come back into another game once you've left that, Darker, and come back to the portal, but go to the opposite side to spend your Elder sigil. Once you've spent your Elder sigil and come into the Darker, run to them for a free contract again, and basically just do them, and you will have chances of actually getting the schematics out of these portals.
In my experience, I tend to have two out of the three in every game I've played. I don't know why it's not three out of three; it might just be my game being bugged or whatnot. I'm not 100% sure on that, but from my experience, that's how it's worked for me. To get all three, I ended up having to do two different games to get all three of the schematics.
So once you've done all that looking at the map, you want to come to just about this area by yourself, and the Xville portal will be there.