Jonas was an inhabitant of Partha. Apparently Partha had also been ravaged by plague in the past. Although Jonas survived, this left his face scarred and pock-marked. During his visit of Partha, Uldyssian made a public display of his awesome powers by healing Jonas.
Jonas then joined Uldyssian's band of followers, eventually becoming one of the more 'high-ranking' individuals amongst the Edyrem. Still, Jonas never really wanted a commanding role; he left that to his cousin Timeon. During the final battle against the Triune's forces in the jungles near Kehjan, Timeon was slain; this event spurred Jonas to take a more active role in the Edyrem's organization, in honor of his cousin.
Jonas fell prey to the ravenous specter of the former High Priest, Malic, during the final battle against the Cathedral of Light. Jonas presumably managed to slay the leader of the Cathedral's forces - the senior priest Oris, who was in fact possessed by Malic - after which he himself became possessed by the specter. Though Jonas was gone, Malic soon followed him into the afterlife.