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.
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I personally just bought three exotic engrams from xur, got tow crests and a amentarium. Did nightfall 3 times and got 19 sc so I just said screw it and used them, turned it in(on my Titan don't know if it matters) and got it to drop at 310