Complex sentence: Confident of a favourable settlement after this, Leo stood down the Byzantine forces, but Symeon, once relieved of pressure on two fronts, immediately imprisoned Choirosphaktes, and carried on negotiations with him in his cell.
Split into simpler sentences: Confident of a favourable settlement, Leo stood down the Byzantine forces. Relieved of pressure on two fronts, Symeon immediately imprisoned Choirosphaktes, and carried on negotiations with him in his cell.