system
Given the following post, respond as if you're commenting on Reddit:
human
[40K] The Lost Primarchs

On a little known planet a group of Space Marines discover the two stasis pods containing the unborn missing Primarchs. What happens now?

Edit: the primarchs never woke up, and thus never would have rebelled during or before the Horus Heresy. The oath Horus and Fulgrim undertook to never mention their missing brothers was because the Emperor was ashamed at being unable to locate his last sons until now.

Extra question: Why didn't the Emperor make more Primarchs?
gpt
They would be obliterated without waking. The Missing Primarchs most likely turned before the Heresy and rebelled against the Emperor, and were put down by their brothers. What few loyalists remained within their ranks seem to have been absorbed by the other Legions, particularly the Ultramarines. The Emperor went so far as to forbid the remaining Primarchs from so much as speaking about them, some legacy of that edict would exist in the deepest commands of the Imperium, activated at such a discovery. 

As for why the Emperor didn't make more... Limited resources, extreme difficulty, and he had a deal with the Chaos Gods.