First of witches are women who has awakened to magic and then after gathering enough knowledge and magic power a stigma is created which in turn transformes their bodies into a spirit body, which means that they become immortal not needing to eat, drink, breath, sleep or ever fear dying of old age. They can still be killed an if they overwork their bodies in a short time they will still get tired and fall asleep.

The reason there have only been female witches up to now is because while men who you could say create the essence of life are able to create mana has no where to store the mana women on the other hand who are built to carry and nuture life are unable to produce mana but are instead able to store and transform foreign mana for their own use. Thus while men can use a little bit of magic because of they constantly generate mana inside of themselves they can never become a witch like a woman can who is able to store large amounts of mana inside of their wombs. Because a witches womb is transformed to carry mana it also includes the drawback that they must or well at least should remain virg*ns before becoming a witch, never explained exactly why but it's implied it's because if they remain virg*ns the womb is transformed to carry and nurture mana instead of a child but if they lose their v**ginity before becoming a witch the womb will instead lean towards nurturing a child instead of mana which will damage their "bowls".

Most if not all witches don't actually create a stigma of their own and instead they are handed down to an apprentice who in turn becomes a witch after the stigma is inherited. This usually occurs after a witch feels that they are unable to progress in their magic. This is both because a witch believes in reincarnation and that she will have another life and shot at magic eventually but also because they all idolize the witch of creation who is said to be the first and strongest witch of all time being able to create life from nothing so they all work to reach her and thus almost all witches act like crazy scientists only focused on progressing their own magic, though they often indulge in worldy pleasures to stave of the boredom that might come with eternal life. Though most witches seemingly chooses to pass on their stigma after a few hundred years.

Lastly witches power is mostly connected to the amount of strokes their stigma holds with each stroke being harder to gain. For instance at 15 strokes a witch gains a passive protection that will protect the witch from anything harmful as long as she has mana while at 20 strokes or above she will be able to even affect the worlds laws like fate, time, space and so on. Gaining more strokes is something most witches are focused on and some are even willing to kill other witches to absorb the magical knowlege hold in their stigmas.

The setting is implied that sometime in the past the witches of Gehenna found Earth and created a portal to cross over after this Earth has become somewhat of a colony of Gehenna you could say from which the witches brings goods and s*aves. Gehenna has with time become more and more dependant on Earth as time went by because of the fact that witches are only focused on their own progress with magic but at the same time both need and wants riches and pleasures, for instance rare mineral and gems are needed as sacrifices for magic while pleasure is needed to stave of their boredom and soothe their minds. So very few witches are willing to actually help other humans or make their civilization progress as a whole, which is why Gehenna is seemingly stuck at around the middle ages but because of portals to Earth you can see people carrying guns or selling modern clothing amongst hemp clothes.

To Earth I would say that witches and Gehenna are a menace, why I say this is because the setting is that the witch of creation to pass on her legacy created a massive amount of homunculus which are magical beasts that after being slain drop a piece of her legacy. However these homunculus awakes slowly with a set amount of them coming alive each year but since this has been ongoing for a long time now they have eventually come to outnumber the witches, what started out as there being 10 witches for every homunculus has now changed to there being 10 homunculus for every witch.

Homunculuses are programmed to protect the legacy they carry but to stay alive and fullfill their duty they need mana and to gain mana they need to attack witches and eat their stigma but becaues of the dropping number of witches which is because most of them are made from inheriting a stigma many stigmas has been lost because of accidents, su*cides and mu*ders. So to gain mana homunculuses has started to eat humans instead to regain a small amount of mana. The more powerful the homunculus the more mana they need for instance one homunculus is mentioned to need to kill and eat 4000 humans to recover after being weakened.

At the same time witches also kill a large amount of humans on their own mostly because of an experiment. On the other hand it is unknown what the witches actually bring to the table though it is implied that the extremly rich and powerful humans on Earth knows of witches and deals with them, mostly to use as mercs against the homunculus though witches are only willing to do that kind of work when they need money to fuel their research of lifestyle.

On a side note the reason why humans don't fight homunculuses on their own is that they move around with a barrier around themselves that makes it impossible for humans to escape, report/inform or even find them until the homunculus is already upon them. The damage inflicted inside of the barrier is also automatically repaired after they leave and the humans killed are turned into mana and dissapears without a trace. So the only trace of a homunculus is if there are a lot of missing person cases in a certain area. They also mostly hang out in densely populated areas so you can't just bomb the place either

All in all the setting at least in my mind is pretty bleak where Earth is sort of invaded by witches but has also become dependant on witches because of the homunculuses. Also the only reason why witches hasn't already taken over Earth in the open is because the strongest witch of them all has ruled that magic is not allowed to be known to the public on Earth because it might weaken some magic and also because a witch should strive to become like the witch of creation and thus it is seen as beneath a witch but the strongest witch will only act if someone truly steps out of line like trying to take over and rule a country in the open or such but it is said that it was 82 years ago since she last acted and whispers has started to go around that she no longer cares....