Demonic Energy is the ‘essence’ or ‘life force’ of Mamono, respectively, and shares a yin-yang relationship with Spirit energy, which is possessed bu humans. Said energy can also corrupt and alter creatures, environments and even godlike beings into demonic perversions, and Monsters can possess far greater levels of energy than humans.
The trade-off is that monsters passively ‘leak’ more of their energy into the surrounding vicinity, and the entities, creatures and people therein, turning normal areas into what are called Demon Realms, and hence why they require Spirit Energy from humans.
Every type of Mamono has their own specific type of Demonic Energy (Kikimora, werewolf, harpy, etc.,) but they also contain part of the energy of whatever Demon Lord is currently in power, and are thus linked to them on a spiritual level. In the age of the former Demon Lord, said link was used by them to help organize and dominate the largely-savage monster hordes, but the current Demon Lord used the link (along with a few alterations of her own) to have her succubus demonic energy alter the bodies of every species, hence why the monsters are now monster girls.
I want to go into further detail, but the rest of the article is at least another ten-ish pages, and i’m not sure i can condense all of it into two or-so.