Cyriacus (Greek Kyriakos, 'belonging to the Lord') was — per the late-4th-century Acts of Cyriacus — a wealthy Roman patrician who renounced his fortune to become a deacon of the Roman Catholic Church under Pope Marcellinus (see Marcellinus's entry above) in the early 4th century. He devoted his time and remaining means to working in the Roman quarries — alongside the Christian prisoners who had been sentenced to forced labour during the Diocletianic persecution — and to ministering to them.
Per the legendary tradition (preserved in the medieval Latin Acts), Cyriacus was famously called by the Emperor Diocletian (or his successor) to exorcise the demonic possession of the Emperor's daughter Artemia. After successful exorcism, Diocletian — instead of executing him — sent him on a long journey to the Sassanid Persian court to perform a similar exorcism of the Persian princess (who was likewise possessed). After Cyriacus's successful exorcism of the Persian princess, the Persian king Sapor gave him imperial protection and gifts (a substantial historical-anachronism — the chronology of Sapor I, Sapor II, etc., does not actually align with the date of Cyriacus's martyrdom, but the medieval legendary tradition disregarded this).
Cyriacus returned to Rome after the supposed Persian journey only to find that the persecution had escalated under the new Emperor Maximian. He was arrested with three companions (Largus, Smaragdus, and Crescentian) and beheaded on the Via Salaria around 304 AD. His relics were first venerated at the catacombs along the Via Salaria, later translated to the small Roman church of San Ciriaco in Via Lata, and finally substantial portions translated to the Cathedral of Ancona (which is named for him as joint patron) and to Bamberg in Germany (where his cult became foundational for the medieval German Vierzehnheiligen tradition of the Fourteen Holy Helpers).