Like the depiction of God, Stuart Slade's version of Satan is a not a supernatural being at all, but an extradimensional alien with superhuman abilities such as bio-electricity, which appeared to early humans to be godlike or demonic beings. However, their natural abilities such as bio-electricity and portal creation meant that technology in both heaven and hell stagnated at a Bronze Age level (and in part because iron is toxic to demons).
Satan was, like in the typical interpretaion of Judeo-Christian theology, one of God's angels, however, he rebelled. Unlike traditional theology, Satan was not cast into hell by God, but seized control of the dimension of himself. Satan conquered and enslaved the native inhabitants of the dimension, beings known as orcs, and then started torturing humans, who's life force naturally transported them across the gap between dimensions after death, in order to gather energy from their suffering. The end goal of this was to gather enough energy to transport a more powerful being, such as Satan, across the barrier to invade other dimensions.
For decades, demons had sent "berserkers" to kill humans on earth for their entertainment as well, the berserkers usually surviving, a notable exception being on killed by Chinese troops in the Korean War. It was not until he invaded Earth after God declared the beginning of the end times that Satan discovered just how powerful humans could be.
Satan sent an army under the demon lord Abigor through a portal to Iraq, however, they were quickly and decisively defeated by the air and armored forces of the the US, Britain, Iran, and numerous other nations. This defeat infuriated Satan, who ordered all of Abigor's family be sent into direct combat.
Seeing no other way to save his family, Abigor surrendered to US Army troops. Shortly afterwards, the multi-national coalitions launched an invasion of hell itself. Meanwhile, humanity discovered how to use psychics to generate portals into hell, and use them to arm humans who escaped from hell's torture. These insurgents cause much chaos through the use of sniper fire and C-4 explosives, and even manage to assassinate a Grand Duke of Hell with well-placed explosives.
The Human armies push deeper into hell after they dig deeper into their arsenal, using carpet bombing, poison gas, and white phosphorous to decimate demonic counterattacks. It is the demon lord Belail who finds a way to retaliate, using naga, a demon race skilled in portal creation, to place portals to Earth on top of volcanoes. Belail orders these portals used to literally rain lava on Sheffield and Detroit, cities told to be human arsenals by damned souls in hell.
However, this does little except to infuriate humanity, who retaliate by dropping four Massive Ordnance Penetrator (MOP) bombs, annihilating Satan's fortress in hell's capital, Dis. The only reason Satan survives is because he is not in the fortress at the time.
Human military forces make further inroads into hell, bombing targets in Dis, as well as Belail's fortress and liberating humans tortured, sometimes for centuries in the "Hell Pit". As his empire crumbles around him, Satan enters an underground bunker and meets with his remaining subordinates. Suddenly a portal opens directly into the chamber and two anti-ship missiles are fired in. Both impact Satan, one in the head, which kills him instantly. A group of US Special Forces, as well as a group of demonic defectors, led by Abigor, no armed with a massive rifle converted from a 30mm autocannon.
Most of the demonic leadership surrenders, except for Belail, who is quickly forced to flee to heaven as human forces surround his fortress.