Legions Imperialis – Adding a Legio, Knight House or Auxilia to your Traitor Legion

And of course, we have to complete the list of all 18 Legiones Astartes by adding those who pledged their allegiance with the Warmaster Horus. And they wouldn't fight alone, not only with other traitor Astartes, but other parts of the former Imperial warmachine.

The listing is a point to begin with, based upon my research in the wikis and Horus Heresy community. The list of allies will probably grow over the product schedule and campaign supplements for Legions Imperialis. While in case of the Collegia Titanica, the Legio Mortis fought along side every Traitor legion of the Astartes, and the only known titan legions who fought alongside the Night Lords, other flexible options are the Legio Fureans and Legio Krytos, who fought with every traitor legion with the exception of Konrad Curzes troops. Further more the support of Traitor Solar Auxilia, Renegade Militias and the Dark Mechanicum was not infrequently.

The 3rd Legion, Emperors Children, first to bear the Emperor's personal standard.

Supported by Titan Legios: Legio Audax (Ember Wolves), Legio Fureans (Tiger Eyes), Legio Interfector (Murder Lords), Legio Krytos (God Breakers), Legio Magna (Flaming Skulls), Legio Mortis (Death's Heads), Legio Vulpa (Death Stalkers), Legio Tempestus (Stormlords), Legio Vulcanum II (Lords of Ruin)

Affiliated Knight Houses: via Fureans (Aerthegn, Hyboras, Perdaxia, Rajha), via Mortis (Devine, Gotrith, Morbidia, Vextrix, Xerathon), via Vulpa (Devine, Ioeden, Niagma), Vyridion

Affiliated Auxilliaries and Imperial Army: Solar Auxilia, Gamonite Theta, Archite Palatines

The 4th Legion, Perturabos Masters of Siege, the Iron Warriors.

Supported by Titan Legios: Legio Argentum (Dread Lancers), Legio Confindo (Sun Cleavers), Legio Fureans (Tiger Eyes), Legio Krytos (God Breakers), Legio Kulisaetai (Gatekeepers), Legio Mortis (Death's Heads), Legio Vulpa (Death Stalkers), Legio Gryphonicus (War Griffons) - Loyalists,

Affiliated Knight Houses: via Fureans (Aerthegn, Hyboras, Perdaxia, Rajha), via Krytos (Caesarean), via Mortis (Devine, Gotrith, Morbidia, Vextrix, Xerathon), via Vulpa (Devine, Ioeden, Niagma), Coldshroud (Loyalists), Hermetika (Loyalists)

Affiliated Auxilliaries and Imperial Army: Solar Auxilia, , Cthonian Headhunters, Castellax Janissaries, Castellax Mamelukes, Selucid Thorakite

The 8th, the Night Lords, Conrad Kurzes Legion of terror.

Supported by Titan Legios: Legio Mortis (Death's Heads), Legio Ulricon, Legio Victorum I + II (Foe Slayers + Foe Hammers)

Affiliated Knight Houses: via Mortis (Devine, Gotrith, Morbidia, Vextrix, Xerathon),

Affiliated Auxilliaries and Imperial Army: Solar Auxilia,

The former Warhounds, the 12th Legion, World Eaters,

Supported by Titan Legios: Legio Audax (Ember Wolves), Legio Fureans (Tiger Eyes), Legio Krytos (God Breakers), Legio Mortis (Death's Heads), Legio Magna (Flaming Skulls), Legio Suturvora (Fire Masters), Legio Tempestus (Stormlords), Legio Victorum I (Foe Slayers) - Traitors, Legio Vulcanum II (Lords of Ruin), Legio Vulpa (Death Stalkers),

Affiliated Knight Houses: via Fureans (Aerthegn, Hyboras, Perdaxia, Rajha), via Mortis (Devine, Gotrith, Morbidia, Vextrix, Xerathon), via Vulpa (Devine, Ioeden, Niagma)

Affiliated Auxilliaries and Imperial Army: Solar Auxilia, Cthonian Headhunters,

The 14th Legion, Death Guard, from the toxic wastes of Barbarus.

Supported by Titan Legios: Legio Audax (Ember Wolves), Legio Fureans (Tiger Eyes), Legio Interfector (Murder Lords), Legio Krytos (God Breakers), Legio Magna (Flaming Skulls), Legio Mortis (Death's Heads), Legio Tempestus (Stormlords), Legio Vulcanum I (Dark Fire),  Legio Vulcanum II (Lords of Ruin), Legio Vulpa (Death Stalkers)

Affiliated Knight Houses: via Fureans (Aerthegn, Hyboras, Perdaxia, Rajha), via Mortis (Devine, Gotrith, Morbidia, Vextrix, Xerathon), via Vulpa (Devine, Ioeden, Niagma), Makabius

Affiliated Auxilliaries and Imperial Army: Solar Auxilia,

The 15th Legion, the sons of Magnus the Red, Thousand Sons.

Supported by Titan Legios: Legio Astorum (Warp Runners) - Pre Heresy, Legio Fureans (Tiger Eyes), Legio Krytos (God Breakers), Legio Magna (Flaming Skulls), Legio Mortis (Death's Heads), Legio Tempestus (Stormlords), Legio Vulpa (Death Stalkers), Legio Xestobiax (Iron Vigil)

Affiliated Knight Houses: via Fureans (Aerthegn, Hyboras, Perdaxia, Rajha), via Mortis (Devine, Gotrith, Morbidia, Vextrix, Xerathon), via Vulpa (Devine, Ioeden, Niagma),

Affiliated Auxilliaries and Imperial Army: Solar Auxilia, Cthonian Headhunters, Prospero Spireguard, Lacunan Lifewatch, Magdan Freekorps, Tupelov Lancers,

The 16th Legion, the former Luna Wolves under the command of the warmaster himself, Sons of Horus.

Supported by Titan Legios: Legio Audax (Ember Wolves), Legio Fureans (Tiger Eyes), Legio Interfector (Murder Lords), Legio Krytos (God Breakers), Legio Mortis (Death's Heads), Legio Tempestus (Stormlords), Legio Vulcanum I (Dark Fire),  Legio Vulcanum II (Lords of Ruin), Legio Vulpa (Death Stalkers),

Affiliated Knight Houses: via Fureans (Aerthegn, Hyboras, Perdaxia, Rajha), via Mortis (Devine, Gotrith, Morbidia, Vextrix, Xerathon), via Vulpa (Devine, Ioeden, Niagma), Malinax

Affiliated Auxilliaries and Imperial Army: Solar Auxilia, Cthonian Headhunters, Ayliet Phalanx, Byzant Janizars

The 17th Legion, Word Bearers, the first heretics.

Supported by Titan Legios: Legio Defensor - Pre-Heresy, Legio Fureans (Tiger Eyes), Legio Infernus / Suturvora (Fire Masters), Legio Krytos (God Breakers), Legio Mordaxis (Deathdealers), Legio Mortis (Death's Heads), Legio Praesagius (True Messengers) Pre-Heresy, Legio Vulpa (Death Stalkers)

Affiliated Knight Houses: via Fureans (Aerthegn, Hyboras, Perdaxia, Rajha), via Mortis (Devine, Gotrith, Morbidia, Vextrix, Xerathon), via Vulpa (Devine, Ioeden, Niagma),

Affiliated Auxilliaries and Imperial Army: Solar Auxilia, 199th Thorian Yeomanry, Ouranti Draks

The 20th Legion, Alpha Legion.

Supported by Titan Legios: Legio Fureans (Tiger Eyes), Legio Krytos (God Breakers), Legio Mortis (Death's Heads), Legio Vulpa (Death Stalkers)

Affiliated Knight Houses: via Fureans (Aerthegn, Hyboras, Perdaxia, Rajha), via Mortis (Devine, Gotrith, Morbidia, Vextrix, Xerathon), via Vulpa (Devine, Ioeden, Niagma),

Affiliated Auxilliaries and Imperial Army: Solar Auxilia, Securran Janissaries

If you want to see it the other way arond and already a Titan Legio from Adeptus Titanicus, you can look up feasible allies for the Traitors here.