The Gerasene Demoniac
This is one of the 13 mosaics of the life of Christ along the upper band of the left wall of the nave. It follows Matthew 8:28-34, Mark 5:1-20, and Luke 8:26-39. In details, it is closest to Mark and furthest from Matthew.
In Mark and Luke there is one Gerasene1 demoniac and he lives "among the tombs" near a hillside with a steep cliff that drops to the sea. The mosaic follows this geography economically, framing the demoniac against a cave in a hill with one side steeper than the other.
In all three accounts the demons possessing the man ask Jesus to send them into a nearby herd of swine. The swine then run off the cliff into the sea.2 Aiming as always for an eloquent simplicity, the artist uses just three swine for Luke's "many" and Mark's "about two thousand."
See also a view of the entire left wall with commentary on the iconography of Jesus and the apostles in these mosaics.
View this image in full resolution.
See more images and commentary on Jesus curing demoniacs.
Photographed at the church by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.
1 In the Vulgate and KJV all three accounts place the event in Gerasa, but modern translations of Matthew have it in Gadara, which like Gerasa is in the Decapolis.
2 Matthew and Mark have mare ("sea") in the Vulgate, but Luke has stagnum, a small lake or a patch of standing water – a more appropriate word, because Gerasa is far from any sea.