The crash of humanity during the end of the DAoT meant a lot was lost to the Imperium when they rose centuries to millennia later. However what if and this is a big IF an area was covered with warp storms right up to the time of 40k (say before the 13th Dark Crusade) maybe several dozen sectors in size. At this time the warp storms abate enough for the Imperium to get into it only to face a section of space using intact STCs, naval forces, ground forces and weapons that not only are from pre-imperium days at the height of their power but with increased technology that fifteen plus thousand years allowed them to do. What are the consequences of the contact at every level of the Imperium and the newcomers and of course what happens next on both sides?
The IoM rape-stomps all enemies with the vast wealth of technology at it's finger tips. The Adeptus Mechanicus wet themselves when they see the might of a STC and the God Emperor is raised from the throne. Then There Is Only WAR!!!!! (and victory for the Imperium)
Situations somewhat similar have happened before, such as contact with the Squats, who had continued to develop their tech since the DAoT, and that high tech world that Macharius hit with a big rock. It'd really depend on the specifics of the DAoT remnant, such as if they're still using full AI, and have they developed a method to safeguard it or just been lucky. Assuming no deadlocking issues, you probably get an alliance, with the remnant being an independently governing associate of the Imperium, like the Squats, Marines or AdMech.
Two real possibilities, I'd say. One: The new area of space is immediately invaded in force and crushed. The Adeptus Mechanicus pulls every string it has and establishes an immense crusade, backed up with all the wealth and technology it has at its disposal; and it recovers the complete STC. The STC is disseminated via the High Altar of Technology, and the Imperium is at a substantial advantage in the future. Two: Some sort of bargain is arranged whereby the Dark Age remnant becomes part of the Imperium but remains semi-autonomous. The Adeptus Mechanicus obtains the complete STC in trade or negotiation, or if the Dark Age remnant refuses, they steal it, by hook or by crook. In both cases, one of the biggest changes to the Imperium will be that the Adeptus Mechanicus will become much more powerful. The political ramifications should be very significant, and I would expect both Administratum and Ministorum to do everything they can to clamp down on this and limit the Mechanicus' expanding power. That's a potential risk. If the Dark Age remnant uses strong AI or is understood to be subjugating humanity to the service of machines, then there won't be any compromise.
Do not forget that the Warp storms could taint much of the remnant technology and people with different ideals.
IN the short story collection 'deathwing', there is a story centered around a explorator discovering a semi-barbaric planet... with the majority of its inhabitants roaming warlords...the sole exception being a monastic order dedicated to preserving KNowledge. WHo use their saved ancient knowledge manuscripts as leverage to preserve their order as the imperium take over (along with a judicious assaniation to show they aren't pushovers.) the twist at teh end of the story was the 'ancient library' was actually a cover for the true source of the orders knowledge... a hidden information based STC... (and it could be implied it is linked with Terra somehow to... since the Order knew of the Imperium and how their society would act despite the planet just having been rediscovered.)
There are many possible outcomes. If they are still using AI, the IOM would invade and would proceed to get their asses kicked so hard the Emperor himself would feel it. Even without AI, they would still be wary as hell of the IOM. Even if they joined it, I doubt the would be willing to outright give the STC away. Likely they would join in as a subgroup that competes with Mars, resulting in a lessening of its influence. That's if they don't have a large degree of worlds or fleets. If they did, then. . . It's entirely possible that they can conquer the IOM with STC technology. Just think of the horrific warships they are able to mass produce. They may not have Space Marines, but when they are slaughtering your ships before they can land, it doesn't matter much. Furthermore, with the kind of equipment they can give their infantry, even Space Marines would have trouble, and they will be able to replace losses far faster than the Marines.
Its never really stated just what the tech level pre-Imperium reached, but as it seems that the high end stuff like Baneblades and Imperator Titans were at least common then, then it would vastly improve the Imperium's war making capabilities.
Golden age technology plus 15,000 years with which to advance means they likely have the ability to do pretty much what ever they want. There only limiting factor would be that they possess a small population base and would likely be unwilling to share there tech with the IOM. Honestly I would love to have a group like that officially added to the universe as actual white knights with out any of the grim dark assoiated with warhammer.
