Maxentius

On September 28, 306, Maxentius was proclaimed Emperor of Rome. From the article:

"Maxentius (278-28 October 312) was Roman emperor from 5 May 311 to 28 October 312, succeeding Galerius and preceding Maximinus II.

Biography
Marcus Aurelius Valerius Maxentius was born in 278, the son of Maximian and later the son-in-law of Galerius. While he was regarded as his father's crown prince during his reign as Roman emperor, Maxentius never held any military posts under either his father or Diocletian. In 306, he rebelled against the rule of emperor Galerius in Italy and took power as emperor in Rome, becoming sole emperor on Galerius' death in 311. He styled himself Princeps invictus ("Undefeated prince"), opting not to style himself either Augustus or Caesar to avoid conflict. However, he took on the title Augustus after defeating Flavius Valerius Severus' siege of Rome, and he added the Alps and Istria to his territory. In 312, his rival Constantine the Great led an army to besiege Rome and claim the empire for himself, and Maxentius' army was defeated at the Battle of Milvian Bridge. Maxentius drowned in the river while attempting to flee with his shattered troops, and Constantine proclaimed himself emperor."