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human
[WH 40K] What are some of the best theories behind the two missing Primarchs?

Bonus, and the other(s) not accounted for?
gpt
Some Imperial scholars theorise that one of the Primarchs fell like a twin-tailed comet unto a world hosting a relatively undeveloped barbarian society alongside a sizeable native Ork infestation, a not-insignificant Eldar presence, Chaos taint, some elements of abhuman ratmen and "beast" men, and some kind of bio-organic Necron presence. According to what data we've gleaned, the Primarch was raised by a tribe of barbarians before rising to its chiefdom and further uniting an entire region of tribes. He drove out the regional Ork warboss and Chaos raiders. He left for parts unknown sometime into his reign, and we have yet to rediscover his presence, or whether he was a Primarch at all.