I heard this piece on Catholic Radio and was intrigued by the blend of chant, organ and musical layering, counter-melody and the unexpected keys involved. The basic chant is the same one we use at Mass during Lent and Advent, but with added keys, voices and parts.
This Agnus Dei, Requiem is used during the Mass for the Dead.
Agnus Dei, Qui tolis pecata mundi, dona eis requiem translates into
Lamb of God, who takes away the sins of the world, grant them rest.