Agathonicus was a 3rd-century resident of Nicomedia who traveled to Greece to spread Christian teachings. During the persecutions ordered by Emperor Maximian, he and several companions, including Zoticus, Princeps, and Theoprepius, were targeted by the imperial governor. After being apprehended, the group was moved toward Byzantium, facing severe mistreatment and death along the way. Ultimately, the remaining survivors were taken to Selymbria in Thrace, where they were executed by beheading.