September 15th is, according to Wikipedia (though I confess I haven’t looked for confirmation of the date), the anniversary of the death of the Eastern Roman emperor Constans II in 668. He was became perhaps the last Roman consul in 642 (the honor has been claimed for his father Heraclius Novus Constantinus, but E. Stein has argued that it is probable that it rightfully belongs to Constans: see I. Shahid, “The Iranian Factor in Byzantium under Heraclius,” [Dumbarton Oaks Papers 26 (1972) 293-320], p. 317 n. 84, and p. 295 n. 3 for the full reference for Stein’s article. He was assassinated while in his bath in Syracuse. As narrated by Paul the Deacon, Historia Langobardorum 5.11-12 :
Sed et alia multa et inaudita harum regionum populi sunt perpessi, ita ut alicui spes vitae non remaneret. Nam et vasa sacrata vel cimelia sanctarum Dei ecclesiarum imperiali iussu et Grecorum avaricia sublata sunt. Mansit autem imperator in Sicilia ab indictione septima usque in duodecimam; sed tandem tantarum iniquitatum poenas luit, atque dum se in balneo lavaret, a suis extinctus est.
12. Interfecto igitur aput Siracusas Constante imperatore, Mecetius in Sicilia regnum arripuit, sed absque orientalis exercitus voluntate.
J.B. Bury, in A History of the Later Roman Empire from Arcadius to Irene, gives a narrative of the reign of Constans II in vol. 2, pp. 281-307 (available online at archive.org). In his account of the death, based on Theophanes, he writes:
In the year 668 Constans was assassinated at Syracuse in the baths called Daphne. A certain Andreas, the son of Troilus, went into the bath with him to wait upon him. As the Emperor was preparing to smear himself with Gallic soap, Andreas, seizing the vessel in which the soap was contained, struck him on the head with it and fled. When the Emperor tarried long in the bath, his attendants, who were waiting out-
side, rushed in and found him dead. (pp. 302-3)