The travler is basically the god that they worship. When it first showed people began living much longer and disease stopped altogether. This ushered earth into "the golden age." Technology improved, people began moving to other planets and began colonizing those planets. Then come the fallen. They originally worshipped the travler as their god and felt that humanity had stolen it from them. So in order to protect itself (and by extension humanity) it created the ghost to seek out fallen people to revive people of potential to defend it. This in turn created the risen. They figured since they couldn't die they became warlords. Eventually new people became risen and they decided to put a stop to the warlords. They called themselves the iron lords, which were the the first of the actual guardians we have now. So all in all traveler created ghost, ghost created guardians, guardians are supposed to defend the tower.