GATE:Thus the Imperium Fought there

Falmart

As The Legions or chunks of it gathered in the chamber with The Primarchs Infront of them waiting for orders.

"As You all know These humans tried to invade the imperial palace with weapons and tactics from a feudal world,Since they declared War on The Imperium and since they Striked First we shall Gladly Return the favor,We will March Beyond the Gate and spread the imperial truth to those on the Other Side, And It is My honor to Lead You, My Sons again since my entombment on the Golden Throne Over 10 Thousand Years Ago.WILL YOU ACCEPT ME AS YOUR COMMANDER?!" Said the Emperor

"WE ACCEPT YOU OUR EMPEROR AS OUR COMMANDER!"

The Voices of the Astartes Were not of each individual but as one. Their voices boomed throughout the palace and it even reached outside the palace.

The Emperor for the first time in over 10 millennia was leading a crusade but this time Humanity isn't alone they were joined by The Eldar in this crusade.

A BattleFleet was Formed above the skies of Holy Terra Preparing to Warp Jump(Since they haven't built a webway gate to falmart yet they're traveling the old fashioned way)
With Each legion Carrying a transponder that emits a Signal only a Eldar or human Star ship being able to detect it, They could theoretically Jump to any part of the universe (and/or other dimensions) with this technique.

It has been decided that only half the legions summoned will only go through the gate. The other half will wait until the webway is Finished The First ones who will go through are:

Lion El' Johnson and his Dark Angels
Magnus The Red and his Masters Of Change
Alpharius and Omegon and their Alpha Legion
The Imperial Fists Led By Rogal Dorn
The White Scars Led by Jaghatai Khan (Bikes included)
5 Death Korps Of Krieg Regiments
2 Cadian Regiments
2 Catachan Jungle Fighter Regiments
One( And it will stay that way) Sly Marbo 
Over 100 baneblades
2000 leman Russ Tanks
(Aerial Forces will come with the BattleFleet)
The Sisters of Battle
The Emperor Leading his Golden Bois

Roboute Leads the Imperium for a while and the ones who'll help him lead are:Corvus,Leman and Vulkan and their legions

The Reinforcements:

The Black Templars
The Lamenters(Needed to join because they always got their Asses kicked)
The Charcadons
The Raptors(The Chapter not the Failed Project)
Iron Fists Led by Skost Ferrus Mannus
A few million Guards men (From various Regiments),
Over ten thousand Tanks(assorted)

"My Lord Isn't this a bit overkill?"Said Kitten

"Captain General,There is no such thing as overkill,Just over preparation."

"Well there are 20,000 astartes Sent to this... falmart"

"Oh,That's because once we take control of the planet on the other side we're gonna take their Galaxy and according to The Human-Aeldari Treaty, They're gonna get half of the Galaxy we're about to take."

"Oh,Okay then,So the Imperium of Man is just technically Married to The Eldar?"

"Now I think about it...yeah."
_______________________________
Troops have amassed on the Imperium's side of the Gate and are preparing to go through:

The Plan Is that
The White Scars Will go first and secure exactly 3 kilometers of land and lock it down and then Kriegsmen and Imperial Fists will go through and create fortifications and Defences
After that The Rest will go through,With The Alpha Legion and Eldar creating a Spy Network and monitor the "Saderan Empire" and troop movements. With knowledge Received from one of the Captured Commanders Called "Count Formal" they knew exactly how many Towns were under The saderans and how big is their empire how much manpower they had and how strong is their empire and then there's mythical creatures....