The monsters should probably be completely organic for the most part, so by carrying pods, it would be cool if the pod was also a living organism or part of the organism doing the draining. Maybe they have glowing sacs on their body that fill with energy.

A lot of them are not intelligent so will just be acting on instinct, although they could understand basic commands by the higher ups.

The sole Class S that we know of..dun dun dun!!!

EDIT: The Goddess might have some of her energy still stored in the center of her chest to keep her alive (yet still in a deep sleep), concealed from detection or impenetrable by even the S Class monster at the moment. Perhaps some of the plot could be centered around getting to that source of energy. Maybe they need the power of that energy to communicate with the destroyed earth and their own dark goddess.

I may be able to post the OOC in a day or two.