Hagia Sophia, also called the Old Bishopric, is one of Nesebar's most atmospheric ruins. Roofless walls, arches, and columns stand in the open air, giving a clearer sense of scale than the smaller intact churches nearby.
Come early or near sunset if you can. The ruin is popular, but in quieter moments it has exactly the melancholy grandeur you want from an old Black Sea church: stone, sky, and the memory of a town that was once far more important than its size suggests.