One emperor's law project outlived his empire by a thousand years

In 529 CE, Byzantine emperor Justinian I ordered legal scholars to compile centuries of tangled Roman law into a single coherent code. The result, the Corpus Juris Civilis, was largely forgotten in Western Europe until rediscovered in the 11th century, when it became the foundation of the civil law systems still used across most of the world today.

— commissioned by Justinian I, Corpus Juris Civilis — completed 534 CE