The series is based on a Christian cosmology, with God, Satan, angels, demons, etc. At some point in the future, humanity captured an injured angel, but they didn't know what they were dealing with, other than it being a powerful extraterrestrial being. Scientists experimented on it and eventually used it as the core of a superweapon called Anthrax (similar to Jojo, many of the characters and powers in the series are named after bands and musicians). Anthrax was basically an artificial kaiju powered by the captured angel and controlled by a supercomputer. It had abilities like flight, teleportation, regeneration, bending space around it to deflect attacks, etc. It could boil entire oceans with its breath and release shockwaves more powerful than nuclear explosions.
As as you can probably guess, it turned on mankind and wiped out civilization. It was eventually defeated by a hero using something called the Dragon Knight, which was essentially a magic/divine mecha. The battle lasted for days and reshaped the surface of the earth, erasing all familiar continents and even changing the earth's orbit. Anthrax was eventually defeated but not killed and sealed away.
As a result of the battle, nearly all of humanity died, and most history and scientific knowledge was lost. Humans had to rebuild from scratch. Also as a result of so much magical power being unleashed on Earth, magic became a much more common thing, and fantasy beings like elves, dragons, and undead started appearing. That's why the setting seems like a generic medieval fantasy at first, as it takes place hundreds of years after the apocalypse.
The protagonist, Dark Schneider (I hesitate to say 'hero' because... well, the title of the series describes him well) is a legendary and feared dark wizard who was also sealed away, and currently has to share a body with a teenage boy, and in this form his powers are severely diminished, and he slowly regains them over the first part of the series. He has to face his former subordinates and others who are trying to revive Anthrax, so he finds himself on the side of the good guys somewhat coincidentally.
Anthrax is eventually revived, but at this point Dark Schneider has regained enough power to effectively one-shot it with his strongest spell, Judas Priest. However this just releases the true angel form inside it, which was held captive for centuries. The angel's true form is much more powerful than Anthrax, and naturally it's pissed at what the humans did to it, so it tries to destroy the Earth in retaliation. Dark Schneider manages to defeat it and banish it to Hell, but it's later revealed that that type of angel was basically fodder and there are angels and demons that are thousands to millions of times stronger. DS has to descend through Hell to obtain new powers and become a Majin (demonic being) to stand a chance against them. He eventually ends up facing off against the Archangel Uriel, who has become corrupted by Satanic powers and fallen, not only vastly increasing his power but also making him an insane being bent on the destruction of the universe. He and DS nearly destroy the galaxy in their fight, not doing so only because the fight shifts to another dimension (but not before they destroy the black hole in the galaxy's center). DS pulls out all the stops to defeat Uriel, including summoning a much more powerful version of the Dragon Knight, and eventually triumphs. And the series really hasn't continued much beyond that due to the author taking forever to update.
Satan is shown to be much more powerful than Dark Schneider and Fallen Uriel, being able to kill an Archangel with a single punch, and his true form being larger than planets and destroying a galaxy just by flying through it. God is of course even stronger, having created the universe and all dimensions, but there is also a mysterious 'Anti-God' that Satan worships, which is implied to be God's dark counterpart. Dark Schneider is also revealed to be the "Adam of Darkness", essentially the evil counterpart to the messiah/Jesus, the "Adam of Light", and their destiny is to fight and one of them will destroy the other, as they are the only ones who can destroy each other. But DS doesn't care, and essentially tells Satan to piss off, as he doesn't want to serve anyone or be part of anyone's plan.
