Gaius Aurelius Valerius Diocletianus (244-311) was a Roman Emperor from 20th November 284 to 1st May 305. He was born to a Illryan family of low status (his father and himself are even said to have been a freedman!) but gradually became powerful.
He succeeded in organizing the tax system with a high tax rate, enlarging the army and establishing a well-organized bureaucratic government. However, some of his plans didn't work out well-- his attempt to take care of inflation by controlling prices was ignored and the tetrarchic system (four people governing) was successful only while Diocletianus ruled. He also tried to get rid of Christians which is said to have been bloody did not succeed in destroying the Christian community. (Christianity became the most popular religion from 324.)
He is significant for stabilizing the economy and the military of the huge Roman empire which made the empire to survive for another 100 years.
He decided to step aside from the imperial office because of illness. He was the first person to abdicate from his position voluntarily. It is said that his despair and illness made him commit suicide.