First Dark Age

When magic was discovered, people quickly began to adopt it and found it made their lives easier in many ways. Magic had its limitations, however, and continued gains in scientific knowledge made older magical spells  cease to function.
Whether because they loved the power magic provided or just out of laziness and a desire to not have to keep creating spells, a group of magicians destroyed the library of Guines, the largest repository of accumulated scientific knowledge at the time.
Their power solidified temporarily, this group spent the next two and a half centuries stamping out scholarly works and technological improvements however they could, stagnating the world in its progress.