Plague / Epidemic
In a mad bid for power, the necromancer Ketior Fulgur spread disease and undeath across the northwestern parts of modern Gallica. He wandered from village to village and city to city, raising the corpses from publically accessible graveyards to rampage across the countryside, stealing wealth and arcane knowledge.
The foul sorcerer was slain multiple times but refused to actually stay dead. Ultimately the people of gallica were victorious, but in response to this crisis the local king declared that all corpses shall thereafter be cremated, to prevent them being reanimated by necromancers. This created a cultural custom that spread over time and is maintained to this day.
Ketior Fulgur was not the last power-hungry necromancer to threaten the world, though few would be as powerful as he was. This would not be the last necromancer crisis, merely the first and the origin of many of the subsequent ones.