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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*puts on spinmetal hat* That mission tenebrous tunnels has you go through mars and into the black garden where you fight a mix of taken hive and cabal. The twilight Garrison has the same characteristics of how a cabal thrusts through the air. The ultra you kill at the end of that mission is a cabal by the name of bracus sho'rgun*? This also is the last story mission you played before the 23rd in which you claimed in another post you achieved the armor before the 23rd. I think this is linked.