User blog comment:DirgeOfCerberus111/Adeptus Custodes Vs. Grey Knights/@comment-33731050-20150327035801/@comment-33731050-20150327080831

The Custodus did fight during the Crusade/heresy. But afterward they stayed with in the palace. I'm not sure why they fought in a battle in the 41st millennium. But I can only see that as either one of those things that are cannon that should not be, of maybe since it was on Pluto in the Sol system that was close enough to allow them. I've never heard of any other cases of them being outside the palace afterward.