Eusebius of Nicomedia (died 341) was the man who baptised Constantine the Great. He was a bishop of Berytus (modern-day Beirut) in Phoenicia, then of the See of Nicomedia, where the imperial court resided, and finally of Constantinople from 338 up to his death.
Read more about Eusebius Of Nicomedia: Influence in The Imperial Family and The Imperial Court, Relations With Arius, Political and Religious Career, Death and Aftermath