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WARNING THIS IS PARAPHRASED FROM MEMORY:
The story started off with an Explorator expedition sent to some backwater planet, upon landing they discovered a very large technologically advanced door (I thought necron at this point... how I was wrong), The explorator sends for drilling machines and breaching servitors (civvy ones I assume), it takes several days to break into the vault.
Upon entering signs of battle are everywhere, but no bodies (think Aliens), he continues to explore the Vault finding machines that are old, very old, and very advanced, he also comes across a load of massive tanks containing a strange liquid, 20
IIRC, one however still has a body inside, the rest are empty, he checks the body for life signs and sadly its dead, so being a good scientist he orders a Autopsy.
During the Autopsy he notes that while this looks like a space marine, its much larger, has a lot of organs that should not be there or he has no idea what they are, the fused ribcage takes 2 servitors to prise open and breaks several saws, the "marines" muscle mass is much much stronger than a standard marines, he estimates 40%
IIRC, he also notes that its healing ability is much stronger than that of a normal marine.
He also finds logs that one of the "marines" escaped, its not clear if the facility was attacked or these marines were the attackers and thats how they got out.
He sends a signal to mars explaining all this....
A while later another research team arrives and finds the entire facility destroyed.
At the time it was theorised by the fans that this was a second primarch project, or part of the cursed founding, but we will never know, these days the Primaris are around so it could just as easily be a test facility for that, but unlikely as Primaris had not been thought of back then, it could be retconned however to allow Chaos versions of super marines, or even some other darker purpose.