You just have to decrypt it from an engram. I didn't get it from a secret quest and am keeping it selfishly to myself. I got it from one exotic engram. However, I've heard other titans have gotten 9 alpha lupis and still not a Twilight Garrison, so my guess is it's drop rate is screwed up and I'm lucky. Just know it's amazing and I can't wait for my titan bros to experience the amazingness of the evade when the drop rates fixed. EDIT: xVoodooMasterx just got it from an engram drop from Darkblade. I've reports of Nagotelli getting it as well. Maybe the drop rates are changing.