Between the fourth and sixth centuries, in the wake of the Christological controversies associated with the so-called "Arian crisis," Rome witnessed the coexistence of Christian communities of differing theological orientations. In certain cases, this situation either resulted from or led to the election of rival bishops for the same city, beginning with the well-known case of Liberius and Felix. Such episodes generated fierce disputes over Christian urban spaces at a time when the topography of the classical city was undergoing profound transformation through the construction of the great Christian basilicas—structures that themselves became focal points of intra-Christian conflict. It is within this framework that the episode examined here takes place. In the context of the double election of Ursinus and Damasus, a Roman basilica—identified either as the Basilica Sicinini or the Liberian Basilica—became the site of the killing of one hundred and thirty-seven Christians gathered there for liturgical celebration. While the Liber Pontificalis remains a fundamental source for cases of double election, the reconstruction of this episode of intra-Christian violence requires engaging not only the Collectio Avellana but also the "pagan" account of Ammianus Marcellinus. The convergence of these sources reveals the religious and political motivations underlying the violence, set against the broader transition from "pagan" to Christian Rome.