In the area of ​​Paracheloitida a low hill south of the village Mastros rises the Mastro Diocese, which is dedicated to St. John ‘Riganas’. According to the prevailing view here must have been the seat of the bishop of Acheloos. Originally the church was built as a three-aisled basilica with a raised nave with three semicircular apses and narthex. He had to plan the shape of oblong rectangle and divided into three naves with adjacent walls bearing arched openings.

The monument belongs to the temples which form the link between the old Christian basilicas and basilicas of the Middle Byzantine period. In the apse stands Platytera between two full-length archangels with imperial uniforms holding globe. On a smaller scale, kneeling, a vaguely distinguished clergyman, possibly a votive. The murals of the church must be from the 12th to 13th century. Small fragments of the mosaic floor of the church with early Christian period features, survive until today in the central nave and the floor of the niche of the sanctuary.