The “Devils You Know” achievement is the second of three UC Vanguard faction achievements in Starfield - by now you’ll have unlocked the Supra Et Ultra (10G) achievement (if you’ve started the faction questline, that is), and you’ll unlock this one when you finish the sixth of nine UC Vanguard faction story quests, titled, “The Devils You Know.” 

Complete “The Devils You Know” faction quest and you’ll only be a few missions away from unlocking A Legacy Forged (30G) achievement, and finishing the UC Vanguard faction questline - and you can finally buy that new house in New Atlantis then!

If you still haven’t joined the UC Vanguard faction, then check out our Starfield - How to join the UC Vanguard faction guide.