Those pictures inside heads probably represent telepathic images . . .

Yes. . . Looks like the non-humanoids suggested to the natives to remake them in their own image. They truly believed that this was the only sign of an advanced civilisation. But the native humanoids refused . . .

Then the invaders decided to uplift the native humanoids by force . . . These are the devices they used to induce the mutation . . . Seems like there's a lot of them over the planet!

And our archaeologist here must've been digging right where the battlefield was. That's why the multitude of skeletons and weapons. . .

So the main generator is in the cave that the robots were sealing down with stones . . . And it is still active!

The natives fought bravely, but they could offer no resistance to the powerful enemy, and were ultimately turned into these spheres . . .