they came in an overwhelming mass

Toward the end of the 21st century they came in an overwhelming mass, from nowhere. We couldn't see their ships, huge shell-like structures hovering in space and outside the visible spectrum. What we didn't know at the time was they were coming home.

They were an ascendent branch of the mollusks that had developed intelligence, manipulated their environment, built a culture, and eventually left the planet as a consequence of an encroaching ice age that was destroying their kelp and shrimp gardens. This species of cephalopods, like the ones we knew, could master their environments by transforming to appear as a twig, a leaf, as part of the background, like nothing at all. This is how they came to us.

We didn't know that they had been among us at the time. They could blend in so well to their surroundings. Then, when their numbers were strong enough and well placed, they began to appear.

Our society fell virtually overnight.

Only pockets of us survive, mostly spread out in various hinterlands, surviving on the local land. We have to choose high ground because the ceph's terraforming operations are beginning to change the world that we once knew. They are flooding the planet by adding ice collected from outerspace. Now that electric light no longer whites out the night we can watch their operation in the glow of the night sky. A dimply glowing ball that appears in the sky at a size that dwarfs even the fullest and brightest moon. It is a very large craft that doubles as their aquatic biome. There, the ceph's spawn much as the ones we know do here on earth. And with that water they arrive in the billions.