This is the Kyrie, or opening movement, from Louis Vierne's "Messe Solennelle" (Solemn Mass) Op. 16 for two organs and choir performed at the world famous Cathedral of Notre-Dame de Paris where Vierne was organist of the Grand Organ from 1900 until his epic death at the console of this instrument in 1937.

In this work "Kyrie eleison" (Lat., "Lord have mercy") is repeated throughout by the choir alternating with solo passages for the organ.  In the largest parish churches and cathedrals in France there were typically two organs, a smaller one (Orgue de choeur) located down front in the east end which routinely accompanied the singers and a much larger one (Grand Orgue) situated high in the west end tribune just beneath the rose window.  Both organ parts in this score also may be performed on the Grand Organ, either with choir or as an organ solo.

Vierne's Mass is a masterpiece, it doesn't get any better than this, and this Kyrie is one of the most beautiful pieces for organ and choir ever written.