Seiryoku

Seiryoku (Japanese: 性力 Seiryoku), also known as Eros (Japanese: エロス Erosu) outside of Japan, is the main source of energy from which all females draw their supernatural powers from. Only biologically females are capable of harnessing the mystical powers of seiryoku, though such a feat is not limited to humans and so, in theory, any species belonging to the same world has access to seiryoku. As a general rule, the amount, type, and quality of one's natural/innate seiryoku is determined hereditarily from birth, though there are multiple ways of gaining or losing more seiryoku.

Combat

In combat, this is what powers most spells and special techniques that require more than just physical energy to execute. Seiryoku users whose moveset specializes in vore tend to have a focus in offensive capabilities in combat, while seiryoku users who specialize in non-vore scenarios tend to focus in defensive aspects of combat, most commonly for self-defense. Self-defense against predators is a fundamental aspect of combat, so those with a higher mastery and understanding of their own seiryoku have higher odds of escaping from an encounter with a predator or even tip the scales and eat the predator themselves.

Vore

In vorish scenarios, seiryoku is the fundamental energy predators require to use superhuman skills that lets them capture, supress, or devour prey. For example, the simple act of being able to swallow people whole by stretching one's jaws costs some amount of seiryoku and those without sufficient seiryoku are unable to do so, though natural predators are likely to always have the bare minimum for executing this action for as long as they are alive. In order to obtain more seiryoku, predators simply have to absorb the life energy from their prey. Since this sometimes also means that the predator directly assimilates the prey's own seiryoku, if any, it is possible for the predator to effectively steal some traits, abilities, and spells from their prey.

Non-vore

Seiryoku is also very useful to pregnant individuals. In a world where there's a constant threat of a fatal encounter with voracious predators, individuals who would otherwise easily fall prey to them may harness their seiryoku as a form of self-defense. Outside of vorish scenarios, expectant women who have high amounts of seiryoku generally have curvier figures, as well as enhanced fertility. Coincidentally, hyperpregnancy is most common among females with high innate seiryoku, and high amounts of seiryoku correlates with increased belly, breast, and/or butt size. The seiryoku that originates from pregnancy also tends to have a focus on utility, which usually includes aiding the user in their daily lives or helping other pregnant individuals. For example, some pregnant spell casters are able to cast strong barriers to protect themselves against enemies. Some other pregnant seiryoku users are also able to create familiars or imbue items with seiryoku, giving them magical properties to aid them in daily tasks, like lifting heavy objects or acting as a guide if the user's belly is so massive they have a hard time seeing things in front of them. Much like predators, pregnant individuals may obtain more seiryoku from eating, though unlike predators they obtain it from consuming "normal" food. It is similar to how herbivores draw their energy from eating plants and vegetables while carnivores obtain theirs from eating meat. It is also possible to obtain more seiryoku as gifts from higher entities, like deities and gods, though this is limited to very special individuals.