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Gaius Aurelius Valerius Diocletianus Augustus (c. December 22, 244 - December 3, 311) was the 51st Emperor of Rome. More commonly known as Diocletian, he was responsible for one of the last great purges of Christians in the Empire.

Diocletian became Emperor on November 20, 284. Feeling the Empire was too large for one person to rule alone, on April 1, 286 he named Maximilian as his co-Emperor. Diocletian took charge of the eastern half of the Empire, while Maximilian was in charge of the western half. In 293 two more junior Emperors were appointed, and each man took charge of a quarter of the Empire.

In 303 Diocletian and junior Emperor Galerius began a great purge of Christians living in their portions of the Empire. By the time it was finished a number of Christians had been martyred. The purge was unsuccessful, and within 25 years not only did Emperor Constantine reverse all the edicts against the Christians, he had also made Christianity the preferred religion of the Empire.

By 305 Diocletian was in poor health. On May 1st he announced that he was retiring, becoming the first Roman Emperor to voluntarily abdicate. Because of his acts against the Christians, Diocletian's successors demonized the man. In Serbian mythology, Diocletian became known as Dukljan, an adversary of God.