I don't know if you already know the answer, but I'll comment my answer anyway. In F/SN:UBWF/SN:UBW season 2, they revealed that Archer's true identity is Hero Emiya Shirou - the MC's future self.
Archer regrets that he became a hero, so he tried to kill the current Shirou (Archer's past self). When Archer used UBW to try to kill Shirou, the MC copied it using the 'trace-on'thingyon' thingy so all the things inside the UBW was copied. I believe it includes the Rho Aias.