Apparently the fallen have 'fallen from grace' and seemed to be a once great empire.
Which sounds a lot like what happened to the humans; what if the fallen empire had been hit by the darkness (5th race, pyramid ships .etc), and are trying to get to the traveller to revitalise their old homeworlds?
In which case I will feel really bad for killing em :(
-
I assert a counter theory to that. They were one of the previous races that the traveler tried to use like us and that the power corrupted their good intentions and they sought the power of the darkness as well as the traveler. The traveler then abandoned them and they turned on each other. They now chase after the traveler out of revenge for their ruined civilization and the darkness is the closes thing they could call a partner.