cf. ps.-Zonaras 656.1 (partial quotation, stopping at e)kleipi/an). The quotation stems from Josephus, Jewish Antiquities 19.273, describing the emperor Claudius' punishment of the two tribunes who had murdered his predecessor Caligula (Cassius Chaerea and Cornelius Sabinus).
[1] The noun sugxw/rhsis commonly means "concession" or "agreement" (see LSJ s.v.), but here something closer to "act of retreating" seems needed.
[2] Josephus' original has the compound sunomwmo/tas "those who had sworn with him", that his, his fellow-conspirators against Caligula.