Before I express interest, I would wonder if you'd take twin primarchs (i.e. like Alpharius and Omegon) scattered to Mars? one would remain there inducted into the Martian cult, another would go off on an explorator expedition around the same time as the unification war. They (at least the one that stays behind on mars) would likely be among the first primarchs rediscovered but thats up to you, could be cool that it isn't even realized they are primarchs until much later