Saint Demetrius was born to a wealthy, noble family and raised Christian. He was a soldier and a Deacon. He was raised to the rank of Duke of Thessaly by the Emperor Maximian. But when he was found to be a Christian he was arrested and imprisoned in a bath-house. He was run through with spears c.306 at Sirmium (in modern Serbia).

St. Demetrius was extremely popular in the Middle Ages and was reported to have appeared during a battle in 586, centuries after his death to help defend Thessalonika. 
Over 200 churches in the Balkans are known to have been dedicated to him. His relics were said to emit holy oil. 

@Evangelizo