Having already been ordained a bishop, Leucius wanted to undertake a missionary apostolate in Brindisi, to liberate the pagans there from misconceptions of Christianity.

The cult of St. Leucius spread throughout the region of Apulia (where many of the rural parishes still bear his name), and he became much venerated in Trani, Lecce, Benevento, Caserta, and Capua.

Later that century, the remains of Leucius, which had begun to attract the attention of many pilgrims, were transferred to Trani, placed in a chapel located under the cathedral.

In Atessa a legend grew around St. Leucius in which the Bishop of Brindisi killed a dragon that had long terrorized the people, and in witness of his work gave him one of the ribs.

Theodosius, bishop of Brindisi and Oria, succeeded in the late 9th century to retrieve parts of the relics of St. Leucius and placed them in a new basilica above the ancient martyrium from where they had been taken to Trani.