I Have been Read the Thunder Warriors for quite Some Times

There is also a Fanfic where the Survivors have Emerge

But i never Saw any where the Thunder Warriors have never been Purge in Unification Wars

So in the Lore that have Said that the Thunder Warriors are Larger and Stronger than the Space Marines, and only the Custodes are the Ones who surpassed them

Even the Thunder Warriors Lacked Some Gene-seeds like the Space Marines Healing Factor (Larraman's Organ)

They could still Fight Long despite being already Wounded and have Past in their Prime, yet they were able to stand Their Ground before dying through Blood Loss, but still Fight Harder to the Death, as they have Know that they are beyond Saving, only Death that awaits them

So, i made a Decision

What will Happened if the Thunder Warriors have given a Change to Fight alongside the Space Marines During the Great Crusade?

Not Only that, they are also Given Gene-seed and they even Resiev Early of the 3 Gene-seed that Are upgraded to the Primaris Marines

Because of that

The Thunder Warriors are now tall as the Custodian Themselves, as well as Strong, Resilient, Fast and Acute Senses  as the Custodes being 9 ft tall without Armor

as well the Discipline, Impressive Will Power, Determination, and hypno-indoctrinated mental safeguards  that the Custodes and the Space Marine have and their Long Lives Biologically Immortal

and Added to their Resistance to psychic attack, with this all, it will be Very Difficult to Any psykers to Challenge them manifesting as a golden aura that seems to make psychic compulsions have no affect on them, and giving telepaths headaches for trying to read them

Warp Sorcery as well, be bad idea, Since it even just makes them Stronger Even more (In the Lore, they go Berzerk, that even Put Pre-heresy Angron Surprise in his Lucid Moments)

and it some Cases, some Thunder Warriors Grow up to 10 ft tall as well even stronger than the Custodes, That which have Put them a Rivalries between Both of the Ancient Super Soldiers as both have been Served the Emperor in unification Wars, But the Thunder Warriors have Done the most brunt in their Wars as they are always Numerous than the Custodes

Even Valdor himself the First Custodes, Possibly the Very Super Soldier that the Emperor have Created Noted to be Taller than the Average Custodes, and the Same Height as Primarch Alperious

And Arik Taranis is the First Thunder Warrior, noted to be very Powerful Warrior in his own Right, Both him and Valdor predates the primarchs

and i Also Added another Gene-seed for the Thunder Warriors

(Sorry but i kinda making of Names) Called the Rage Within

Like the (Belisarian) (As i said it was given to them Early) Furnace (Only Furnace)

During Combat Stress, This Gene-seed Will Make the Thunder Warrior Even More Stronger, Faster, and Tougher to be killed (Their Curse is not Included)  and would likely Worked in Tandem with the Belisarian Furnace if they are in Death's Door, making all the Way Harder to be Killed

While their Curse the Rage of Vengence, is After Primarch Ronan Himself Saw his First Lover Died at the Hands of his Enemies

It is Similar with the Black Rage of the Blood Angels, But it Boosting the Strength, Speed, their Resilient and their healing Factor Much More than the Black Rage

and they can still Focus themselves of which is to kill as they still know Friends from Foe

This Was Only been Used if a Certain Thunder Warrior Wanted to kill their Enemies, or See their Friends being Killed and would even more Determined to kill their Enemy

Because of this, Not even the Arrogance of the Custodes will ever Dare to Challenge them, even if it's a Sparring,  as some of more Experience Thunder Warriors have Known to Channel the Rage of Vengence and use it, in any Opening moments or they have lose their Strength and Needed some Boost

Due to them being the First Super Soldiers in the Front lines

They are able to have the Same Tech, Weaponry, Armory, Vehicle and Dreadnoughts as the Custodian Themselves much to their Ire

And Luckily only their Elites and officers could have this Equipment

Only the Praetorian Guards, Ronan's Honor Guards Themselves able to have this Equipment

and it has been said that the Praetorian Guard Themselves such is their skill could Challenge the Experiance and Veteran Custodes to a Fight, if Given a Chance, Something that the Space Marine Won't do even if they did, they would Surely lose to a Custodes

Next are the Brotherhood of Champions, This is an Entire Division of Thunder Warrior Champions in their Respective Division (Equivalent of Space Marine Chapter) (Similar to Grey Knight Paladins) They wear Terminator Armors (Any Pattern) Or their Trusted Thunder Armor, This is the Elite of their Respective Divisions

The Division which is the equivalent of the Space Marine Chapter, they Numbering of 10,000 to 25,000 Thunder Warriors Led by the Division Commander (Chapter Master) each one divided into Company which is 1,000 Thunder Warriors led by the Captain and Demi-Company is Lead by the Lieutenants 

The Thunder Legion is at Best is a jack of all Trade and as well good in adapt to any Situation

As Ronan has the Traits of his Brothers, Like Guilliman (Statement, Strategist, General and Logistics) Lion El Johnson (Warrior, General and Secretive) Sang (Charisma, Warrior, General) Vulkan (Friendly to Humanity, a Smith) ETC

In the Lore the Proto-Primarch is Called The Angel

He is the Best Example of, What if the Primarchs are being Raised by the Emperor?

he is Controllable at First, But then Become a Monsters as he saw every Humans or Creatures is unworthy of his Father's Love

and So i Make the Proto-Primarch like his Younger Brothers to be Scatter as well

His Ship the Might of the Imperium Or Humanities Dream

Ronan Found the ship after the Unification in his Adopted Planet

The A.I Itself have Claim that the Ship Exist the Start of the Dark Age of Technology, with the Tech of both the Golden Age and the Dark Age of Technology

And it has still the Weapons and Vehicles STCs that is still stored in it's Data Banks

and only the Thunder Warriors Have Ever Access to this Wonders of Technology, and would have share this Tech to the Young Space Marine Legions, But due to Heresy, Ronan is hesitant to Share this tech, even so, he will surely keep the Best for himself and his Legion

(I'm still Searching for other Advance Scifi Tech that will be gonna Used in the Future Chapters)

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So someone have as well Suggest to me to include the Perpetuals

I'll looked for some Possibility of how i introduce them

As for the Empress, well she is still Maybe Usage the Sailor Moon

(Unless you have suggest to me, another Benevolent Goddess, we have already too many Badass Woman we Gonna need a Benevolent goddess)

As for the Crossovers well be Difficult for me due to i have to re-write again

Well if you have Read my Other Crossover Stories

then if i do

Maybe Just simply Mention that they have travel to the other Galaxy that i already Write,

Maybe they will have their own Version of Gate Story

Disclaimer: I Do not Own Warhammer 40k, they Belong to their Respective Company