What you read here can shake your very trust in the Emperor and the Astartes. If you prefer to live in the lie and belief that humanity's finest warriors are flawless, then stop reading. If you have seen the film Ultramarines and think that the Ultramarine Chapter is the Emperor's finest, then I will hereby prove you wrong.
What do we know of the Ultramarines?
Well, we know that they were the 13th Legion to be created among the 20 first founded Legions. Their Primarch was Roboute Guilliam, the very author of Codex Astartes.
Currently The Ultramarines control nine star systems. This makes their Chapter unique among its peers.
It is said that Horus sent the Ultramarines Legion away to a distant part of the galaxy prior to the heresy. This caused the Ultramarines to miss the battle of Terra, and therefore they were at full strength when the war was over.
At this time Roboute wrote his famous Codex and enforced it upon the others. He even used firepower to force Rogal Dorn of the Imperial Fists into submission. You could say that he gave Dorn an offer he couldn't refuse.
When the Tyranids "invaded" Ultramar, the Ultramarines got to prove their place in defeating them with the aid of a Titan Legion which is Ultramar's planetary defense. So when all legions were forced to split up, the Ultramarines got to have their own Titan Legion in their backyard? And just because the Ultramarines are so adept at killing these so-called aliens (Tyranids) they get to send a lot of marines into the Deathwatch.
Ever thought that these Tyranids might be a bio-morph project run by the Ultramarines? So that they can justify their Titan Legion and so that they can put their men into the Deathwatch? This smells.
Finally... I have found ancient files that the Imperium has tried to destroy. In these files the following is recorded:
"Chapter Ultra-Marine of the Legiones Astartes was founded during the inter-legionary wars of the thirty second millennium. Tradition places the date at 4001001.M32 - the very first day of the millennium. The chapter is therefore over eight thousand years old, making it a chapter of the third founding. Upon its inception, the Emperor gave the chapter the number 13 - formerly the number of one of the treacher-legions now banished to the Eye of Terror 'without number and name with all honours erased'.
Along with their number, the new chapter received the gene-sperm, implant zygotes, rituals, and other paraphernalia of indoctrination previously entrusted to the banished 13th legion. The chapter's founder was Roboute Gulliman whose bones now lie in the Reclusiam on Macragge"
- White Dwarf issue 97, January 1988
Now this really smell. The Imperium is clearly trying to make us believe that the Ultramarines are tru first founding marines. But in fact they are nothing but replacements. Is it even verified that it is Roboute's bones that lie in the Reclusiam? Can it be so that Roboute is still out there, alive and kicking, scheming and building his power base?
My belief is that the Emperor's true finest - the Dark Angels, actually has discovered this coverup and are secretly picking off stray Ultramarines (nicknamed Fallen) in order to interrogate them so that they can collect more evidence of this whole business of Roboute's. That is why the Dark Angel's secretly is building a power base of their own, so that when the day comes, they will be able to bring Roboute into justice...