There's a really good theory going around in Warhammer 40K circles about the Custodes.
So like, they don't wear armor and have black clothes to honour the fallen emperor, right? And they are all stationed on Holy Terra alongside him, right? Yet they all have literally the best of the best of his equipment with them, and are the strongest warriors of the Imperium, right?
Well, what if they weren't twiddling their thumbs. What if, just under the surface of the palace of the Emperor, there is a massive warp gate or eye or something, constantly spewing daemons and shit due to the proximity of the Emperor, and the Custodes are literally down there, 24/7/365/40k, fighting the shit out of them and risking their lives in glorious battle, and the only ones you actually see at the palace are resting from the constant fight they endure.
I mean, they are literally the best, it makes sense they are doing the hardest job.
It's not official, but it's official in my heart.