The most important church on Agios Achillios island (see: www.ipernity.com/doc/294067/47395972) was the Basilica of St. Achilles. The church was founded at the end of the 10th century by Tsar Samuel of Bulgaria. Initially it was the cathedral of the empire of Samuel and later - until the middle of the 15th century - a bishop's church.

In the basilica the mummified remains of Saint Achilles - a bishop from Larisa in Thessaly - were kept. These were brought to Agios Achillios after Tsar Samuel conquered Larisa in the 10th century. At the same time he commissioned craftsmen from Larisa to build the three-aisled basilica, which became the seat of the Bulgarian Patriarchate. The impressive church - (20 x 47 meters ) - was a cruciform basilica and among the ruins today one can see the famous sanctuary with three lobed windows, the pillars made of bricks and stones and a tomb on the south side.

After the reoccupation of the area by the Byzantines in 1018, the church became dependent on the archdiocese of Ohrid. In 1072 it was plundered by foreign invaders, but it continued to function until at least the middle of the 15th century. Over the years the church was abandoned, deserted and finally became a ruin.