Saint Thaddeus, Apostle of the Seventy, was by descent a Hebrew, and he was born in the Syrian city of Edessa. The holy Apostle Thaddeus of the Seventy must be distinguished from Saint Jude, also called Thaddeus or Levi (June 19), who was one of the Twelve Apostles.

When he came to Jerusalem for a feastday, he heard the preaching of John the Forerunner. After being baptized by him in the Jordan, he remained in Palestine. He saw the Savior, and became His follower. He was chosen by the Lord to be one of the Seventy Disciples, whom He sent by twos to preach in the cities and places where He intended to visit (Luke. 10: 1).

After the Ascension of the Savior to Heaven, Saint Thaddeus preached the good news in Syria and Mesopotamia. He came preaching the Gospel to Edessa and he converted King Abgar, the people and the pagan priests to Christ. He backed up his preaching with many miracles (about which Abgar wrote to the Assyrian emperor Nerses). He established priests there and built up the Edessa Church.

Prince Abgar wanted to reward Saint Thaddeus with rich gifts, but he refused and went preaching to other cities, converting many pagans to the Christian Faith. He went to the city of Beirut to preach, and he founded a church there. It was in this city that he peacefully died in the year 44. (The place of his death is indicated as Beirut in the Slavonic MENAION, but according to other sources he died in Edessa. According to an ancient Armenian tradition, Saint Thaddeus, after various tortures, was beheaded by the sword on December 21 in the Artaz region in the year 50).

Source:

https://oca.org/saints/lives/2017/01/04/100025-apostle-thaddeus-of-the-seventy