This will be a very uncommon occurance, a few chapters per faction(mainly warhammer and other factions) fighting, if you want to put any suggestions on how wrong i am, then i will judge if you are right or not, but do not expect me to put this out reguarly, just kinda whenever i feel like it

Rules

Warp will not have any effect exept for pschers and warp engines

NO chaos gods

No star wars eu

Las gun is equal to standard blaster, hell guns are about equal to a shadow trooper weapon

Force users are classified as pshykers and can be dealt with as such(blanks etc)

Warp drives can go 2 the speed BUT 25 percent chance of somthing happening(getting there before they left, being destroyed etc)

No rebels

Time:41st millenium before cadia is destroyed and before gulliman and the primarius are unveiled(50-60 years before), and before death star 2 was destroyed

locations:WH40k will be located in the unknown sector, no previous contact

Prep time-none

Contact:small patrol fleet comes into contact with imperium patrol fleet, pyyranic victory for imperium but only 3 cruisers are left.

Imperium:finds out about potential lost human colony, sends investigation fleet with a dispatch of the cult mechanichus to look for STC's and to help maintain the fleets wargear

Empire:becomes awar of lost fleet and sends prob to investigate, finds large fleet docking at border world and sends a fleet to reinfroce surronding worlds and another one to attack

1 battle:imperium fleet sees large fleet remenisence of the fleet from earlier, caught off gaurd the imperium loses ΒΌ of their ships before attacking back, another pyyranich victory and the crusade is put off, while the raven gaurd and kreig troopers come to reinforce, and an mechanichum exploration fleet comes and 1 emporer class battle ship

Empire:loses most ships but 3 jump out and inform empire, major renforcments come and reinforce the worlds surronding the battle, darth vader comes with his personal fleet and a few inquisotors(now known as dark inquisitors for empire and inquisitors for imperium).

5 months as reinfocments come

First battle:the main forces go to the planet nier(made up) wich is where vader stationed his fleet, massive a battle in space while spce marines and imperial gaurd are deployed onto the surface, raven gaurd partakes in hit and run tactics disrupting communication and defences while kreig troopers assult the main city of condor. And the horrifics of the cult mechanichus creations are severly demorlaizing to defendin soilders.

Space:imperium wins, but is again a pyyranich victory, and vaders ship is still around along with most of the empires fleet they are just taking pot shots for a while, imperium loses multiple ships and 13 iornclads and the emporer class ship is heavly damadged and returned to imperium space.

Ground:heavy ground fighting, with the kreigs innumerable numbers and the space marines hit and runs along with shock tactics getting the imperium lost of ground early on.

Empire forces are able to defeat and even push back multiple attacks, due to the 501st legion on the ground and defensive positions, but each victory for both sides results in numeros loses of troops and equipment.

second battle: request for reinforcments are sent and are sent

In 5 months

Imperium:lots of new ships and 1 more emporer class battle ship and 2 ultra marine companys(including CATO SICRIUS(JK))

Emps:starts to regain strength but will take a long time till he is able to move or think anything more than a few simple sentances, so 18 years till being able to talk(even then only 16 word sentances) and 100 till able to be fully functoning(at least)

Empire:more rienforments are sent and death star 2 is sent to subjigate the invaders, emporer palpatine is starting to get force visions and is starting to mobolise the empire to war.

Notes:imperium still thinks that it is just a rebbeliouse lost colony, so is not sending all forces, emporer will not be able to do anything in this war, but will slowly regain power and imperium is currently winning, but with massive casulties(for them) but when they find out about the rest of the empire they will start to mobolise most forces towards it

The empire will be curbed stomped unless they are given some advantages, so they have death star 1 and 2, and palatine gets a bit of a head start to mobolise all forces to fight the imperium.