Xerius was the Romulan spokesman for the Elysian Ruling Council, located at Elysia, where he spoke as one of the elders on the council alongside Devna and Magen. Xerius became lost in the Delta Triangle many centuries prior to the 2260s, when he was transported to Elysia.
Xerius greeted Commander Kor and Captain Kirk upon the arrival of their ships, the IKS Klothos and USS Enterprise to Elysia in 2269. He explained to Kor that Elysia was a curious trap, where time passes very slowly, and that he, as well as the other Council members were centuries old, despite looking young. He further explained that those who inhabit Elysia have made the best possible world there because the have found there is no escape from it.
As the crews of two ships refused to believe they were stuck in Elysia and attempted to escape, Xerius reminded Devna, who felt their attempts were futile, that it was not against their laws to attempt an escape, nor should their attempts to escape be impeded, as it was ultimately their choice, even if it meant death.
After violence broke out between McCoy and Kaz, during a reception aboard the Enterprise, Xerius ordered Kirk, Kor, and the two belligerents to the Elysian Council Chamber. In the Council Chamber reminded Kirk and Kor that all forms of violence were forbidden in Elysia, and because Kaz started the fight and attempted to kill McCoy, he proposed that the Klothos and her crew be frozen for a star century. Following a plea from Captain Kirk, as the Klothos was needed to aid in their escape, Xerius released Kor into Kirk's custody.
Later, when the dual-ship attempted to leave Elysia, Magen sensed the Klingon's deception and sabotage of the Enterprise, prompting Xerius to warn Kirk, moments before the two ships returned to the Delta Triangle, and normal space. (TAS: "The Time Trap")