28. May 2021
Added new short story.
This is the phase of taking over the planet Earth and eating all - or most - humans.
At around the year 2034, the few perennials that were still in their home dimension, and the ones who are already in ours, can't wait any longer. The last few also enter our dimension and the takeover begins. This takeover is a war. After they have infiltrated most of human society and our governments, and gained a following, they strike.
During the first stage of Phase 4, the initial strike knows as the the Wild Hunt, the tank heads start a bloody revolt. Their goal is to take over the planet and enslave humans. This means that they can become the new ruling species and turn the humans into their livestock. They are to be eaten to ensure the immortality of the perennials. The humans naturally don't want that to happen and fight back fiercely. Even some tank heads join the resistance forces and turn on their kin.
At the time of the takeover, it is not completely scientifically proven why consuming humans works to extend a perennial's lifespan. However, they have found that eating other lifeforms like herbivores or other animal carnivores/omnivores doesn't quite get rid of their hunger as efficiently. Plants don't work at all. The theory is, that tank heads are at the very top of the trophic food pyramid and thus would have to eat "lesser" lifeforms all day long to even get close to gaining the energy they need to survive. While something like a wolf or shark would probably sustain them for a short while, they argue that these food sources are not as easily accessible and too rare in comparison to humans to be a true alternative.
The resistance is actively trying to figure out alternatives and doing their best to not starve in the meantime.
Back in their home dimension, the perennials were star-like creatures. To survive, they had to feed. A perennial who doesn't feed dies and turns into the equivalent of a black hole. In their star-form, this is achieved by consuming other perennials. For a long time, they didn't think anything of it; after a sort of big bang where they were created, there was plenty of them around. But eventually, the smartest ones realized that this lifestyle wasn't sustainable.
In this ecosystem, everyone was a predator. No one was safe from being eaten. In fact, they consumed each other faster than they could produce new offspring. Not to mention that consuming another perennial was dangerous because it could end in a disastrous fusion or fail. They decided that they had to find another way of surviving.
Scouting out other dimensions, they soon found ours and found it suitable for their purposes - consuming the life there.
A perennial who enters our dimension can be killed by disease or injury, but this only destroys their temporary meat body. Their true "essence" returns to their home dimension when this happens, from where they can simply return with a new body.
Of course this process takes some effort and energy, so unless they make sure to feed upon returning again, they may become too weak from multiple entries to attempt another one. Their fate in that case is to stay in their home dimension and either feed there or die.
Eventually, their new food source will probably be depleted, unless the perennials manage to keep a stable human population alive through whatever means. They are of course concerned about this future at the time, and different "camps" have formed. Some want to start a breeding program of sorts, others insist that they can just find a new dimension and eat all life there once their food runs out in this one.