Complex sentence: The Byzantine Empire was greatly offended by this and worked towards re-establishing either a joint patriarchate or a single Greek patriarchate; the terms of the Treaty of Devol in 1108 nominally restored a Greek patriarch, although this was never enforced.
Split into simpler sentences: The Byzantine Empire was greatly offended by this and tried to re-establish either Greek patriarchate or a joint patriarchate. Though the Treaty of Devol in 1108 nominally restored a Greek patriarch, the treaty was never enforced.