Supposedly they are the forces of the darkness/entity. The pyramids themselves seem to be ships. As for what is controlling them or who is on the inside, nobody knows. Not sure exactly which one. But if my understanding is right the traveler has kept them at bay. by force twice now. Once at the beginning of the collapse/ end of the golden age. And once at the end of season of arrivals. But recent lore entries suggest that the pyramids could attack again but are waiting for an unknown reason. Possibly due to savathuun somehow. So the reason for them just sitting there in a ring around our solar system is 100% unclear. We are completely surrounded.