Once, in the dark and dreary past in the time before current recorded years, a great reckoning occurred in the world of magic- the goddess of magic, Mystra, was slain for some time, and thus all magic became distorted and crazed throughout the multiverse. Entire universes simply disintegrated themselves due to Mystra's death, and those that survived were sent careening into the abyss known as the Mage Wars.
These wars were essentially battles for power or to slay certain insane wizards, and they raged all throughout the multiverse in the bloodiest time in all of history. The Mage Wars were so catastrophically large that other wars that occurred were merely small spin-offs of the ongoing Mage Wars- wizards atttempted to destroy all other wizards due to some innate craving inside of their arcane skulls for power in the vacuum of such power.
The death of Mystra had other effects on reality aside from destruction and mayhem- magic had become unbalanced, with raw energy surging all throughout the infinities of reality. This caused one of three things to occur within every single universe in reality- one, it became highly dense with magic, making it possible for nearly any person to take up magic given a few minutes and an eldritch-enough looking stick; two, the universe became unhinged from its normal laws and remains so to this day- these are known as "Mutaverses", derived from "mutare", to change. They are highly fluid in their laws and most can be altered by significant enough willpower alone. Finally, several universes became inhospitable to magic overall- arcane deadzones, some the size of entire planes of existence and others only small areas within them, sprouted up everywhere, where something like a surge of magic had drained all energies there. Anti-particles of arcane might destroyed any magical abilities within those spaces- barbarians tended to rule those lands, driving out the now-powerless wizards.
The Mage Wars by and large led to the destruction of most civilization at the time- and while they were thousands of years ago, their scars still rake the world's core today. Few positive outcomes occurred in the mage wars; most knowledge was lost in the sands of time as wizards slaughtered their entire civilizations until some form of renewal was needed. And, after Mystra's return and balance returning to magic, the wars slowly subsided, with their billions of casualites.
And yet, despite these horrific memories from millenia ago, they are beginning again. Perhaps more quietly this time- but the fabric of the Spell Weave is being tampered with by the Man in Black himself, circumventing the might of Mystra, and changing the laws of reality. And this has led to factions being formed- those who align with the Man in Black and those who don't. Entire wars have already begun as something like a preamble to the Second Mage Wars- the War of Guardian City, the War of the Hell Furnaces, and of course, the Dragon Wars, are all but small facets of the Mage Wars and its many ugly faces.
Prepare yourself for the worst...