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The twenty '''Space Marine Legions''', also known as the '''Legiones Astartes''', were created by the [[Emperor]] to take part in the [[Great Crusade]]. All the [[Space Marine]]s of a legion (''Battle"battle-brothers'' " amongst themselves) were modified with help of the DNA samples of a single [[Primarch]]. As a Primarch was found, he would receive the command of his respective legionLegion. His homeworld would also become the new homeworld of the legion Legion itself.

The Legions were massive armies, and the size of each could vary tremendously. A precise number was never truly achieved and maintained. Even during the Great Crusade , some legions Legions were very numerous, while others were not. The numbers would always vary with new recruits and inevitable battle-losses, and also important was the availability of potential recruits and the administrative skills of the Primarch and his officers.

The most numerous legion Legion of all was the [[Ultramarines]]. The [[Thousand Sons]] of [[Magnus the Red]] were of a small number as many of them had developed mutations or uncontrollable levels of psychic powers. [[Fulgrim]]'s samples had been largely lost, and this left the legion Legion of the [[Emperor's Children]] also with a very small number. Both of these legions Legions would increase their numbers to acceptable standards only after their Primarchs were found.

One should not forget that the legions Legions were for a very long time under the direct command of the Emperor and that originally all recruits were from [[Terra]]. The '''Terran Marines''' would always remember these times and the legions Legions had Terran traditions. Later, as the Primarchs were found, the new recruits would have very strong loyalties towards them, and some of the Primarchs would rename their legionsLegions. The Emperor saw no problems with this development; the loyalty of the Primarchs to him was believed to be unshakable. This reasoning would be proven to be sadly mistaken.

The first Primarch to be found, [[Horus]], was the Emperor's favorite and he and his legionLegion, the '''"Luna Wolves''' ", were even granted the honor of being re-named renamed as the "[[Sons of Horus]]". The '''"First Legion''' " was given the new name ''"Dark Angels'' " by Lion El'Jonson, the '''"Dusk Raiders''' " received the name ''"Death Guard'' " by Mortarion, and the '''"War Hounds''' " received the name "World Eaters", by Angron, and while its original name is unknown, the highly honorific name '''"Emperor's Children''' " was awarded by the Emperor only '''after''' a speech made by their Primarch Fulgrim. It is unknown if other legions Legions were also re-namedrenamed.

Half of the known legions Legions revolted during the [[Horus Heresy]].

|colspan=7 align=center style=background:#4F4F4F align=center|'''The twenty Twenty Space Marine Legions of the Great Crusade'''

! colspan="6" |unknown Unknown - all All records lost following the Horus Heresy.

! colspan="6" |unknown Unknown - all All records lost following the Horus Heresy.

|align=center|unknownUnknown

|align=center|roughly Roughly 60% of all chaptersChapters.

|align=center|unknownUnknown|align=center|possibly Possibly [[Storm Giants]] and [[Black Dragons]].

*the The ''[[Mythos Angelica Mortis]]'', which deals mainly with the [[Astartes Praeses]], but also provides information about the Dark Angels and its successor chaptersSuccessor Chapters. It reports that these chapters Chapters refer to themselves as the '''"Unforgiven'''".*the 'The ''Apocrypha of Skaros''' and the '''Apocrypha of Davio''', the first one tells us that there were 23 successor chapters Successor Chapters of the Ultramarines, but fails to name them, the second source is able to provide 8 eight names of them.

==Two unknown legionsUnknown Legions==

While the nine ''"Excommunicate Traitoris'' " Legions are known to the Imperium as the Traitor Legions, of the 2nd and 11th Legions no records remain, including of which side they took during the Horus Heresy. The history of the Dark Angels during the Heresy is similarly known only to themselves, and possibly the Emperor. Therefore, excluding the role of the Dark Angels, there were at least nine, and as many as eleven Legions who took the side of Horus.⁴ For more detailed information see the [[Two unknown legions]] article.

The loyalist legions Legions were later reorganized by the [[Codex Astartes]] and broken up into several 1,000 Marine-strong [[Chapter]]s during the Second Founding.

A single Chapter kept the name, traditions, rituals, homeworld, and original identity of the origin Legion, while the remaining Chapters received a part of their legionLegion's [[gene-seed]], new homeworlds, new names, and over time eventually developed their own traditions and identities. These chapters Chapters are known as '''"Successor Chapters'''", a title which is not granted to chapters Chapters created during later Foundings.

The nine '"original chapters'Chapters", to a certain extent, are more highly thought of. These chapters Chapters are called '''"First Founding Chapters'''"¹ or something similar.

In the current Age of The Imperium, nearly three-fifths of all Chapters are '"descendants' " of the Ultramarines, either directly or indirectly through one of the Ultramarines ' Second Founding chaptersChapters. Many records have been lost over time, consequently there are many Chapters which are unaware of which legion Legion they ultimately descend from.

The [[Grey Knights]] are an exception as this chapter Chapter was created shortly '''before''' the Second Founding and its gene-seed is of secret origin.

From a pure technical standpoint, the [[Chaos Space Marines|Chaos Space Marine Legions]], also known as the '''"Traitor Legions'''", are the only Space Marine Legions still in existence. In reality however, all of them have been broken up to some degree and now exist largely as separated individual warbands following rival [[Chaos Champion]]s. On occasion , a highly successful Chaos Champion, such as [[Ezekyle Abaddon|Abaddon the Despoiler]], is able to re-unite reunite these Traitor Legions once more and unleash them upon the galaxy.

*[[Codex: Space Marines (4th Edition)|''Codex: Space Marines'' (4th Edition)]]*[[Codex: Dark Angels (4th Edition)|''Codex: Dark Angels'' (4th Edition)]]*[[Descent of Angels (Novel)|''Descent of Angels'']] ppp. 235, 303*¹[[Codex: Black Templars (4th Edition)|''Codex: Black Templars'' (4th Edition)]] p. 20*²[[Soul Drinker (Novel)|''Soul Drinker'']] and [[The Bleeding Chalice (Novel)|''The Bleeding Chalice'']] by [[Ben Counter]].*³[[Warriors of Ultramar (Novel)|''Warriors of Ultramar'']] by [[Graham McNeill]].*⁴[[Codex: Ultramarines]] |''Codex: Ultramarines'' (for 2nd Edition), ]] by [[Rick Priestley]], p. 7, "''Of Legions numbered 2 II and 11 XI nothing is recorded, but it seems likely these also fought for Horus at least during the beginning of the Heresy.''"