St. Matthias is the apostle chosen to replace Judas Iscariot (look up Acts 1:21-26). Little is known of his missionary activity, but tradition states that he preached in Judea (see map) (modern Israel), Cappadocia, and on the shores of the Caspian Sea. He was either stoned to death in Jerusalem, or was martyred at Colchis or at Sebastopolis. St. Helen brought his relicts to Rome, and some were tranferred to Trier, Germany (This site is in German, but you can see pictures of the abbey). He is credited with a Gospel and other discourses that have not been historically confirmed.
It’s important to note that, since St. Matthias died in the first century, history can be murky. Some sources claim he died at Sebastopolis. Others say it was in Jerusalem. The relics brought to Rome by St. Helen may be those of another St. Matthias who was a bishop of Jerusalem.