With his position thus strengthened, Theodore expanded his territory into northern Macedonia, although it is possible that at least part of this region had already been captured by Michael I after the death of the local Bulgarian ruler Strez in 1214. It is unclear to what extent Theodore's expansion involved direct conflict with the Bulgarian Tsar Boril (), but by 1217 he held Ohrid, Prilep, and most of the plain of Pelagonia, at least up to Strez's old capital at Prosek, and likely beyond, approaching the Strymon River. As the Greek historian Konstantinos Varzos has noted, the capture of Ohrid, seat of the eponymous archbishopric, was particularly important for the standing of the Epirote state and Theodore's aspirations. Theodore sponsored the election of the distinguished canonist Chomatianos to the archiepiscopal throne in 1217, and Chomatianos would repay that support with his steadfast championing of Epirote claims to the Byzantine imperial inheritance vis-à-vis the rival claims of Nicaea.