If both ships are the same class they could just rewrite the ident codes to and name on the hull to the Invincible Reason, who the heck is gonna question the Dark Angels on their very much known and secretive ways. Not like most people would tell the difference anyway.
Even if it is not the invincible reason, it's still a f$&@ing Gloriana class ship.
Make no mistake We will now around the top of their shit list after this campaign. There's no way for us to actually defeat them for good, because they are recruiting worlds, and most valuable stuff is going to be deep in the imperium with the Rock.
The most we can affectively do here is bloody their nose, and maybe steal some of their shit.
We are going to need to change some policies and harden defenses around the Knights worlds because they will not stop.
My concern is that they might have rallied their successor chapters into this; one chapter, even a first founding one, is a beatable threat. Multiple chapters working in concert, led by the aforementioned First Founding Chapter? A far tougher foe, if it can even be beaten.
Again, I don't see them, pulling in full chapter forces from their successors in this first attempt.
They'll probably have a few companies worth but in my opinion, they are here to kill the Knights .
The prophecy stated the Beowulf sub sector in particular. That means the dark angels aren't here for the entire sector. They don't have the motive nor the resources for that kind of fight (while keeping it privte)
If they have enough information to know exactly where the knights are they probably know of us, and the fact that we are engaged in a separate campaign. Which means they probably won't anticipate us redeploying all of our forces to counter them. And there is no way in hell they are anticipating the tau showing up (both because the tau are new on the scene and their souls are hard to read from the warp)
Make no mistake. This is going to be hard and it's going to be costly. But the DA are also unlikely to be prepared for the sheer amount of f%* we are going to throw at them.