Several billion years ago the Celestials seeded the galaxy with Earthlike life. That's why all the worlds have biochemically similar DNA-based life. Then the Celestials went away... but they left an automated system in charge to watch over things. That automated system is the Corax.
The Corax is basically a biological expert system. It is incredibly adaptable and, in its way, extremely intelligent. But it has no consciousness; it is not self-aware. It's just a system. And it follows a handful of simple rules, like Asimov's laws of robotics.
The first rule is "Obey the Celestials", obviously. That's hardwired in. The second rule is "Protect the Garden" -- the Celestial system of life-seeded worlds. As part of this function, the Corax periodically descend upon these worlds to track and record their progress. That's why they can produce Avatars, perfect duplicates. Every million years or so, they'll send follow-up probes to each planet. In order to collect data, the probes take the form of various local species. On Earth, a Corax probe would shape-shift into a redwood tree or a shark or a triceratops or whatever, and live a complete triceratops life, and then transmit its information back to Corax Central. (Central has a pretty complete record of life on Earth going back to pre-Cambrian times.)
The third rule is "Correct excess deviation" -- prune and trim the Garden, eliminating dangerous problems. And this is where humanity comes in. We suddenly expanded across a chunk of the galaxy, spreading mass extinctions wherever we go, replacing the local flora and fauna with raccoons and roses, house cats and wheat fields and and goats and sparrows. We are /very/ deviant. From the Corax POV, we're a sudden cancerous plague exploding across their Garden.
-- Why were they surprised? -- The Corax are accustomed to moving in geological time. The last time they visited Earth, genus Homo had figured out flint-chipping but was still struggling with fire. Our next checkup wasn't due for another fifty thousand years. It took the Corax a couple of centuries to notice the destruction, and then another decade or two of thinking about it to react. (That's the period when Corax ships were appearing but not attacking.) Of course, now that they're responding, they're deadly in real time.
-- Can't we talk to them? Convince them to stop? -- So far, no. The Corax have no consciousness, right? They have no concept of "other". They don't communicate. They just carry out their hardwired instructions. Trying to talk to them is like trying to argue with Microsoft Grammar.
However! While the Corax have made avatars for millions of years, they've never made avatars of a self-conscious species before. So they are now starting to become infected with self-awareness and individuality. In the short run, this will lead to euphoria, as Corax subystems wake up and discover It's Awesome To Be Me! (We would see some of this in Arcs 3 and onward.) In the long run, it will lead to disagreement, dissent, confusion, and conflict.
-- So we can leverage that? -- Unfortunately, not so much. The spread of individual consciousness through the system will not make the Corax generally friendly. Some instantiations might become less hostile to humanity, but the default response from Corax Central will be, "Huh, humanity is even more deviant than we realized -- it is also a cognitohazard. Kill it harder."
-- So we're doomed? -- Not quite! The Celestials, or at least some remnant of them, are still around. They're hanging out down in the galactic core, thinking post-Singularity thoughts and doing Kardashev II stuff. If a group of humans can (1) get there, and (2) attract their attention, and (3) convince them that humanity is worth saving, then humanity can be saved.
I know how (1) would be accomplished: it's stipulated that for longer Jumps, you need deeper gravity wells / more twisted spacetime. So to reach the galactic core, you'd need to dive at high speed into the ergosphere of a rapidly rotating black hole. Then (2) and (3) would be "play to find out".
And that would be Arc 6, the finale.
