The XIth? You mean the Wardens of Light?
Interesting read, but they're definitely not the guys I meant.
My XI Legion were called the Abyssal Host, although I'm changing the name to The Oceanborn, since I like it better.
I'll try to summarize the Oceanborn as a Legion. They're roughly greek in culture, come from a world dominated by naval battles, and due to their Primarch's influence will constantly tinker with and refine technology to better suit them. They favoured mechanized warfare supported by elite infantry formations, but were generally flexible enough to try their hands at all manner of battles.
Their Primarch, Nerius Argonar, is a tech-savvy and hugely charismatic man with a keen eye for details and a "good enough is never good enough" mindset. Alongside more traditional roads to improvement like refining training routines or devising new strategies, Nerius always has plans to improve each and every piece of gear he gets his hands on, either through calibration, refinement, or tweaking of some details. The XI legion carries a lot of esoteric gear - Nerius has strong ties to the Mechanicum and is often at the forefront of hunting for lost technology, although his tendency to experiment earns him as many enemies as friends amongst the high-ranking Magos.
Nerius was evetually exiled for trying to build better Space Marines and tampering with the geneseed in order to do so, with predictably disastrous results.
Geneseed is little-understood and even small changes can have far-reaching effects - in this case what appeared to be a successful upgrading of half the Legion turned out to be a preliminary stage for complete mental and physical degradation - the marines Nerius experimented on either mutating hugely or outright dying. The degenerating marines cost the Imperium a handful of victories against a would-be counter-empire in Segmentum Pacificus, and worse, allow these usurpers access to Legion technology and the geneseeds of the few unaffected marines left to fight a war alone.
In a fury, the Emperor exiles Nerius and his Legion, to wander the empty void beyond the galaxy for a thousand years as penance, with no man permitted to speak, write or otherwise make mention of the Emperor's Wayward Son or his Legion until such penance is completed. When Nerius attempts to protest, both Horus and Russ are made to basically chase him out, to their joint regret.
The counter-empire was defeated, of course. The last survivors of the XI Legion, imprisoned in enemy territory, were executed by the Space Wolves, a solemn duty carried out with due gravity.
Nerius leaves the galaxy, flying blind into dark space. He gets into a battle with an ant-like alien race called the Axoda Roh who live in the darkness beyond the stars. Destroying and damaging several of their colony ships, the Oceanborn fight defiantly against increasingly overwhelming numbers until their Primarch is killed in battle by a hyper-corrosive toxic sword used by the Roh's vaunted Lord-Alchemitect (their war leader). Nerius, of course, survives long enough to Power Fist the Lord-Alchemitect's head clean off his shoulders before succumbing to the injury.
Afterwards, a single Oceanborn battle barge is steered homeward, bearing two hundred and fifty marines as an honour guard, and the corpse of their Primarch.
The rest of the Oceanborn Legion die fighting in the empty void, warring to the very last to avenge their Primarch.
The battle barge sent home is swallowed by a freak warpstorm on the galaxy's edge, and spat out near an area of space called the Glastheim Rifts...
And from there the Oceanborn undergo the transition from 'Legion' to 'Chapter'. There's more details on how, but I'm saving that for a moment because typing all of the above has made my fingers hurt.
The funny part being I'd had this idea a year or two BEFORE Primaris was a thing. If Conn Eremon was still around, he'd be able to point out how I'd discussed the Abyssal Host and their Primarch with him a few years ago.
I like the mad scientist/Tony-Stark-The-Primarch story enough to keep it, and it also lets me sit here going to myself "Sure, Cawl did ALL the work on Primaris. Yep, definitely. Absolutely NO PART of Nerius' research was involved."
I think that as long as you work out who knows what about the XI Legion heritage, how they know, and why it makes it important, along with any consequences of who knows, then you shoul go for it. There are many, many ways I could see it playing out that it would be interesting to see your take on it.
My current intention is that only the Chapter itself knows the truth about their origins.
Well, them and eventually Guilliman - who has a lot more to deal with than a beaten-up Chapter sharing the name and colours of a missing Legion.
I *might* hint that Big Bobby has paid the Oceanborn a discreet visit during the Indomitus Crusade, but don't know if that's a worthwhile addition yet.
As you put this in Liber Astartes instead of Liber Chaotica, I'm assuming your XI Legion remnants are loyalists. You might as well have the Chapter ID itself as an Ultramarines successor- in The First Heretic, Aaron Dembski-Bowden hinted the pre-Heresy XIII Legion absorbed II and XI Legion members who weren't purged- the only ones who know otherwise are the Emperor, Guilliman, Cawl, and the XI Primarch himself, of whom the first is unable to tell the truth, the second and third are unwilling.
Naturally, the IA should minimize mention of links between your Chapter and the XI Legion, unless there was good reason- maybe the Sons of Malice attempted to force your Marines to join them in worshipping Malice/Malal (whose holy number is 11)?
Actually, the Oceanborn are going to claim descent from the Imperial Fists. There's some circumstances involved here, but largely what they're going to do is take the place of a Dorn-Lineage Chapter that is all-but destroyed by heretics. The few survivors of the IF chapter will instruct the XI Legion on how to behave like a Chapter, for the purpose of ensuring the IF successor is avenged.
I do plan to minimize the links between the Oceanborn as a Legion and the Oceanborn as a Chapter.
For example, if the notion of being XI Legion successors doesn't sit right, I hope to frame the above as the Dorn Chapter simply changing name and colours, cutting ties with their past selves who were so easily deceived and defeated by traitors.
Their fortress monastery - on an island - is going to be bombed into the ocean. Bits of it are going to survive, or at least be salvageable to the point the modern Chapter's fortress monastery is largely underwater. Hence, Oceanborn - those recruits chosen for the Chapter are taken under the wild seas to begin their new life as Space Marines.
Yeah, I think my next step will be to work out a proper outline for these guys and see if the concept holds water (ha!) in anything approaching a proper article format.
This might take a little while, though. It's going to be hard work making it all watertight (ha!), after all.