Basically, it's this:
Necrozma
And Eternatus
Are from the same species
Considering they are both dragons in their final form and the fact that their 2nd type is the opposite of the other
As well as the fact that they are both responsible for significant Pokemon power ups (Dynamax and Z-crystals)
How they have similar anatomy structure (riggedy crystal forms)
And how both of them go on a rampage (one was more responsible before the rampage)
I rest my case
Tell me what you think