Complex sentence: When Turlough was thought to have died, the brothers were freed, but Con left, and without his presence western Tyrone was exposed to a direct attack from Hugh O'Donnell, the lord of Tyrconnell.
Split into simpler sentences: When Turlough was thought to have died, the brothers were freed. But Con left, and without his presence western Tyrone was exposed to a direct attack from Hugh O'Donnell, the lord of Tyrconnell.