Before Grixis split off from the mega-plane of Alara, things were different. The land was called Vithia, a white-aligned empire of humans and other races, a land proud of its dynasty of wise and honorable monarchs. If you wanted to be part of the solution rather than the problem, you lived in Vithia. If you wanted protection from the necromancers and demons who sometimes plagued the outlying ruins and trade routes, you lived in Vithia.
But when the subplane became separated from the others, it lost access to beneficial and growing magic. At the same time that Vithian mages and clerics lost the main source of their magic, they also had to contend with the rise of black mana, and the associated rise in power of the necromancers and demons. Over time, the defenses of Vithia failed. Vithian citizens became prey for the necromancers' growing armies, slaves to rampaging demons, and victims of treachery of corrupted forces within the kingdom.
Strongholds across the kingdom became prey for those mages who practiced necromancy with a military bent. Vithian knights became overwhelmed by, and then part of, the rotting armies. Over time, proud castles became cities of the dead, ruled by necromancer barons, ogre warlords, vampires, or demons.
The surviving Vithians still had a chance, however - they still had military superiority over the mindless minions of the necromancers, and they had something even more precious - they had life. As decay gripped the world, Vithia remained a sanctuary for the living, a beacon of optimism in the increasingly dismal and angel-free world, simply by the magic inherent in the simple cycles in people's lives. As long as the Vithian kings remembered this faith in the fundamental power of life, Vithia would never fall.
The Vithian king known as Sedris was human once. He ruled Vithia as one of its wisest kings in the early years after the plane's separation from Alara. No one knows what triggered him to give in. It's unlikely that anyone alive today even remembers that he once sat on the throne in Vithia's capital. For Sedris was seduced by evil forces. Demons, unable to assault the kingdom directly, instead tempted its leader with dreams of power, and he fell. Sedris handed the lives of thousands of humans in his kingdom to demons in exchange for the power and unlife of a lich.
The kingdom flooded with demons and the forces of the undead. The capital fell, and just as Sedris had changed, the capital of Vithia, too, became something else. Today it's known as Sedraxis, the largest necropolis on the plane, named after the last of the Vithian kings. It's still ruled by Sedris, now a lich warlord, whose mastery of the combined arts of battle and necromancy remains unsurpassed throughout the world of Grixis.
After the corruption of Sedris, the plane became known as Grixis, after an old Vithian word for traitor. The necromancer barons and demon lords conquered more and more of the lands that formerly belonged to the living. As the power of white and green mana ebbed away, the only sources of life that remained were the survivors. Creatures that depended on life energy, such as demons and blood-sucking vampires, couldn't feed on the dead. As the supply of living humans dwindled, their power struggles became more violent. Even necromancers have learned to seek out the precious energy of life, known as vis, to power their spells, causing the surviving humans to seek shelter in remote, ward-covered strongholds called hermitages.
The living humans of Grixis today don't remember much about Vithia, even as they crouch in its ruins. They fight the forces of undeath with cleverness and magic, focusing on the power of red and blue mana to thwart the death magic of their predators. They still call themselves Vithians, the word a tattered reminder of times long past. But it's a losing battle. Every time a human being (or an ogre, or a kathari, the plane's vulture-aven) dies, it becomes a resource for necromantic warlords like Sedris, meaning they add to the enemy even as they subtract from their own number. If something doesn't change, humanity will go extinct on Grixis in a matter of years.