This image shows the sepulchre of St. Petronius, the 5th c. Bishop of Bologna.
His relics however, are actually buried in the church of San. Petronio in Bologna.

This part of the San Stefano church complex is known as San Sepolcro, and was built in the 12th c..
It was modeled after the Holy Sepulchre in Jerusalem.
The sepulcro can be accessed through a door to the left of the altar of another church in the San Stefano complex, Chiesa del Crocifisso.