Byzantine frescos from the 12th century.

On the central panel: ‘The Harrowing of Hades’: the triumphant descent of Christ into Hades (Hell) between the time of his Crucifixion and his Resurrection when he brought salvation to all of the righteous who had died since the beginning of the world (excluding the damned). With his right hand, Christ has grabbed Adam by his wrist, to pull him out. They are surrounded by various righteous figures from the Old Testament (Abraham, David, etc.).
In the Eastern Orthodox Church, the ‘Harrowing of Hades’ is celebrated annually on Holy Saturday (the day before Easter), during the Vesperal Divine Liturgy of Saint Basil.

On the arch: Angels (left), an archangel (top) and Holy maidens (right).

Correspondent: J.M.Criel, Antwerpen.
Source: (amongst others) Wikipedia.