Red Room achievement in Remnant II

After waisting hours of my time running back and forth in the Adventure mode, I found a different way to unlock it if you, like me, won't have the Imperial Garderns in the Campaign mode.
Move to Cathedral of Omens and wait for a blood moon in the sky, as usual. You can move out to Ward 13 or to different places until you get it.
Now move the mechanism to open the door on your right, the one with the 2 treasure chest... just in front of them you'll got the trap door leading you to the blood room.

Just to be sure: be offline.
-- EDIT --
chwzie pointed out that this can be achieved online too (tnx)

Enjoy.