There are a series of terminals throughout Halo 5 that chronicle the journey of an 'undesignated Forerunner Builder'. He is searching for someone or something called 'Bastion', and unwittingly finds himself on Genesis during the events of the game. The final terminal details that the domain is 'repairing' (either him or itself, or perhaps Bastion?), whilst the ancilla and the Warden are distracted... and then he finds Bastion, 'alive'.

So, who or what is Bastion? I'm guessing it's a place, as the way he talks makes it sounds like a last defence... a refuge of some kind. These terminals prove that there are other Forerunners alive out there, so perhaps he is searching for their final enclave. It's worth noting that, I believe, the terminals span the hundred thousand years since the rings' activation, as he talks about both the Domain and the Organon being destroyed after the firing of the Halo array.