That was the first spell. That's what some priests will tell you. But who knows? There were at least 8 different spellcasters at the dawn of magic in the world. They formed schools and taught others what they had discovered. Alteration was the most prolific and the most powerful at first. The school of Evocation had few spells but was dedicated to martial prowess. The young Evokers were constantly daring and dueling. Abjurers, the magesmiths crafted Mage armor most of which no longer exists. Of the lesser schools there are the extroverts: Enchanters and Illusionists and the introverts: Divination and Necromancy. Of course, there were the Summoners. Some say they started it, others are sure it was the Evokers. Either way the Mage War lasted for generations. The Summoners fielded mages with battalions of crossbow wielding Unseen Servants upon whom the would cast Flame Arrow. Then the Evokers developed the Fireball and Lightning bolt spells. The devastation wrought by these spells brought the first MageWar to a close, with a peace brokered by the school of Alteration, who would take the magic users into an age of exploration throughout the world via the Teleport spell or into the astral plane via accidents while using the Dimension Door spell. The illusionists, who along with the other lesser schools were but mercenaries during the Mage war began to increase the scale of their magic. They had always considered themselves artists, they were now treating the landscape as a canvas with the Hallucinatory Terrain spell. During this time the Evokers withdrew from the world, literally walling themselves off from the rest of the world behind fire and ice, iron and stone.
After a time the second Mage war occurred. This time the aggressors were the Necromancers who attacked the Evokers over long held grievances left over from the first Mage war. The undead armies of the Necromancers nearly wiped out the Evokers before, ironically the Summoners newfound ability to Conjure Elementals put an end the the second Magewar.
For some time there was relative peace. During this peace there was friendly competition, even cooperation. The wizards of this time began to see the divisions between the schools as artificial and they began learning from each other. This was a long period of prosperity as well as enlightenment, for with the Diviners newfound ability to Contact Outer Planes humanity was reaching far beyond previous understanding. Eventually a group of wizards, master of Summoning and Abjuration sought to tap the powers of beings from beyond. This cabal of Wizards was able to summon demons and bind them into their service with the Cacodemon, Spiritwrack and Trap the Soul spells. Prosperity continued for a few short years before the demons were able to go free. These demons returned with an army and the third Mage war began. This time all wizards were united against this army of demonic aliens. The Mages of this era bore into the earth creating secret defenses and redoubts from the demons but the demons were able to penetrate and forever polute the magics of these places. It was a Summoner who eventually ended the final Mage war by casting a Wish spell so powerful it ended his life, but rid the world of the demon army.
Now, in the wake of this final Magewar against the otherworldly demons there is an uneasy truce amongst the magic-users of the world aided by the practice of wizards living in isolation in towers in the wilderness. Some chaotic magic-users and clerics (evil high priests) have chosen instead to dwell amongst the demons and monsters in the ruins of the underground fortresses created during the last Magewar.