|This article or section involves Matthew Ward, Spiritual Liege, who is universally-reviled on /tg/. Because this article or section covers Ward's copious amounts of derp and rage, fans of the 40K series are advised that if they proceed onward, they will see fluff and crunch violation of a level rarely seen.|
A "liege" is a sovereign to whom one owes fealty. Therefore, a "spiritual liege" would be a person to whom one is ideologically loyal, like a thinker whose philosophy one followes.
This phrase has come to be widely used among fans of Warhammer 40,000 after Matt Ward gave an interview explaining why Codex: Space Marines had so many Ultramarines characters in them: "[t]he Ultramarines are undoubtedly the best Space Marines ever. Yes, really! Thanks to the heritage of Guilliman and their myriad heroic deeds, the Ultramarines are the exemplars of the Space Marines. With a few fringe exceptions... all Space Marine Chapters want to be like the Ultramarines and recognize Marneus Calgar as their Spiritual Liege." (The real reason was it was going to be Codex: Ultramarines until at the last second GW changed their minds.)
Apparently the heresy of acknowledging someone other than the Big E as their liege, be it physically or spiritually, is lost on the Wardian Space Marines.
Because it makes total sense why they'd venerate the Ultramarines and not their own Chapter Masters and Primarchs.
Since then, "spiritual liege" has been used as an insulting way to refer to Roboute Guilliman, Marneus Calgar, and Matt Ward. It is also used ironically, to make the point that one's spiritual liege is not one of those three. In ages past, the term was used instead of the phrase "Matt Ward" on /tg/ (since if you should ever type "Matt Ward" or "Matthew Ward" in the text box, it'll count your entire post as spam).
In the Horus Heresy novels it's heavily implied that the remnants of the two lost Legions were absorbed into the Ultramarines, after being loyalty-checked into nigh oblivion by the Emperor's Executioners. Therefore a large number of so-called Successor Chapters aren't really Ultramarines but venerate Guilliman over their original Primarch. On the other hand, when your alternative is facing down the Space Wolves, you'd pledge anyone, even Abaddon, as your liege.
Amusingly enough Abaddon is in fact the Chaos Spiritual Liege, as many renegade Chaos Space Marines do renounce their Primarchs and venerate him when they join. Actually absolutely appropriate if Abaddon is in fact somehow a clone of Horus (even if weak weak weak in comparison).
Why this isn't QUITE as dumb as you think
Other than the fact that technically Ultramarines were always viewed by non-Space Marines as the prime example of what it means to be a Space Marine and if it wasn't for Guilliman, the Chapter would not exist...
Ward's work is poorly worded but there's some truth in it. There is rather a large number of Chapters that don't know who their Primarch is, either due to the Imperium's exceptionally professional record-keeping or because it was covered up, often due to the gene-seed used coming from a traitor Legion or being somehow tainted from the start. The lineage of some Chapters could be guessed based upon key traits, but many others are just too "generic". The Soul Drinkers thought they were Imperial Fists successors, and the Blood Ravens are probably
also sons of Guilliman assuming the "loyalist Thousand Sons" theory doesn't pan out (although it probably will) successors to the Thousand Sons (check out this video theorizing about it!: ). Since it's a common fact that Ultramarines are the prime and favored source of gene-seed due to their low mutation rate it is a safe bet to assume the Chapter is one of their successors; Dark Angels successors are irrevocably inducted into "the Unforgiven (basically Legion Building)" without exception, Blood Angels successors all have Red Thirst, the Poets and Imperial Fists both have notable and consistent defects in their gene-seed, and neither the rastas, nor the furries (although the Space Wolves can't actually make successors) have any real habit of forming successor chapters, the former due to lack of numbers during the Second Founding, and the latter courtesy of Fenris nature or Emperor's plan or whatever, so it kinda narrows down the list of who your Primarch could be (specifically, Rowboat, Genghis Khan, and Metal Hands. Even then, both the mongols and the ultimate Mechanicum bros only have a handful of successors while the Ultramarines have a large majority.).
The novel The Unremembered Empire shows the Ultramarines have a number of Traitor Legion Loyalists within their ranks, even saying their battle cry "We march for Macragge!", one notable group was a full Iron Warriors Chapter, notice an Ultramarine successor has similar colors to Iron Warriors? Humorously they've rallied to Guilliman's foster mother.
And if none of that calms you down, just pretend that Ward's codex is just propaganda collected from Macragge that the Smurfs stick up to make their citizens feel better.
It is frequently touted that the Ultramarines have the most Successor Chapters. This is mainly because, in addition to their gene-seed, they were the largest Legion at the time of the Second Founding. However, it is widely suspected that the Ultramarines absorbed the Legions of the two missing Primarchs. Now, it's entirely possible (and probable) that a fair number of these Space Marines would be running around at the time of the Second Founding. So when the Ultramarines legion was splitting up, it's entirely possible that Chapters were made from the gene-seed of the missing Primarchs. Therefore, whilst it's still probable that the Smurfs have the most Successor Chapters, there might not be as many as you'd think... given that it's entirely possible that dozens, if not hundreds of Chapters are actually descended from the missing Primarchs.