Whoops

Where to even begin...ah, let's just start with the situation. At length.

What Didn't Happen, But Everyone Is Pretty Sure Happened:
Once upon a time, the world was at peace, and beginning to expand. For a long time, they had even enjoyed the appearance of beings from another, close by planet, which had not only a common language, but a means of transport quickly established! However, from the depths of the planet, a demon of unrealistic power arose! The demon enlisted the help of many, many demons, and many said that he outright changed many of them into grotesque beasts never seen prior, promising power, which they certainly received. Others have said that he gathered many, in order to fuse their powers in order to create what we know as dragons, which still terrorize the world today. A vast number of demons had unified underground, and thus the combination of man and beast terrorized the world and everything on the way up, destroying the gates to the other planet and bringing it to ruin. And during all of this, Hochmut, the goddess, was nowhere to be seen! (In the end it was revealed that this tyrant was so powerful, it blocked off all communication between Hochmut and her own world, and the connection is still quite damaged today.) However, thanks to an organized effort of a small group of legendary heroes, who had all recognized themselves as the most powerful of their species (one of each earthly being, demon included), made an effort to go down and slay the tyrant, which after a long and epic adventure detailed in storybooks everywhere (with varying accuracy), succeeded! But, the demon of the group turned on the party in an attempt to take the tyrant's power and resume the tyrant's reign itself, and in the end, the demon kills all but the human, and the two proceeded to exchange a cross counter fatal blow. In the end, nothing but the ruins of the battle remained, and attempts to negotiate with the demons that remained only served to prove a point, that demons couldn't be trusted. And as such, the demons were cast away from civilization, and the leftover energies of the tyrant continue to torment the world in the form of yet more monsters. The gates to the other world (and the other world itself, for that matter, save for their impressive tower) remain destroyed, the world unable to flourish thanks to the constant presence of beasts beyond the world, the deeper reaches of the world sealed off, and the world much worse off than it was before. But hey, at least the monsters themselves have powerful stuff to take off of them. Silver linings.

What Actually Happened:
Once upon a time, in the very beginning, a long time before any of the above, there were two gods: Hochmut, and Edelmut. After a series of ultimately minor events, the two friendly (to each other, for the most part) gods decided to create their worlds close together. Edelmut, the more easygoing of the two, made a less demanding, simplistic planet, with only one major species living on it composed solely of mana/ether/magicstuffs, which developed in a simple, yet comfortable way. In fact, she'd chat up people that she made, to see how they were doing, that's pretty much how laid back she was about the whole god thing. Strangely, this was a good thing, for she was thus a quite loved god! Meanwhile, Hochmut had ambition. She wanted to create an incredible world, and went full blast on it, creating various species, giving some certain abilities, varied landscapes, large surface area, etc etc etc. And, well, she succeeded. However, as the world continued to flourish, she realized that she, as a goddess, could not maintain the planet she had created. Not being the most reasonable person due to simply being an unreasonable person and the incredibly uncomfortable issue of maintaining a world one cannot, she created, with dark thoughts clouding her mind, the demigod known to little as Pandora, and went into a slumber. Now, Pandora looked at the world she ended up in, took a good look around, and decided that the demons were the most vulnerable of species, thanks to many living underground, volatile attitudes, something in their genetic structure, so on, so forth. And thus, it created the 'demon tyrant', brainwashed thousands of demons (and manipulated a good deal more), and then went to the other world with the really handy transportation. Hey look, everything here is magic based, including the goddess herself. Just set down a powerful, self-fueling magic draining circle in an unused part of the world, and by the time it's actually noticed by the goddess that runs the place (who hadn't realized anything was happening yet, she wasn't the most perceptive person), it'd be too late and it'd have a very nice pick-me-up for the one that created her. Back to the planet, making sure the gates were wrecked, check up on the demon tyrant...and it was still alive, but some folk looked like they might actually slay it. But that's OK, that was planned. Let them kill it successfully, barge in, gloat, kill them all, make believable shit up and brainwash some people to believe it, and then brainwash some more demons to ruin negotiations, then mess with the mood to ultimately completely ruin the demon's chances at living normally. With the link to demons and monsters achieved, the creation of monsters, the vastness of the amounts of them, not to mention their power, and progress, and perhaps civilization itself, was sent. the death of billions outweighed the power of the monsters, and thus Hochmut, together with the pick-me-up of the energy of most of an entire planet, woke up, decided this was acceptable, and also decided it wouldn't be a good idea to keep up chats, lest people get too curious. And so, the world progressed to how it was.

Sure, Pandora isn't all that subtle, but it certainly got the job done! World is shit, and Hochmut's ability caught up, leaving her even more alive and well than ever before. Edelmut, on the other hand, was reduced to her essence, and nothing more, for as long as a god's planet exists (after making one), the god cannot die. Sure enough, that means pretty much infinite energy, and so she's being used as a battery, buried deep away. There's life on the ruined planet, but not much, for monsters ended up there, as well. Monsters that certainly don't need magic (but find it delicious), preying on what remains in the ruined tower (the only place that the circle couldn't chew through by the time Pandora came back), or each other. Lovely situation!