Mors et resurrectio is the first one of Langlais' "Trois paraphrases grégoriennes". (The second one is Ave Maria and the last one is Te Deum , and the corresponding uploads are put together in the playlist below).
It depicts the struggle of life and death
Mors et resurrectio has, as inscription, a citation from a epistle by St-Paul "Death, where is thy victory ?".
It begins pp in a kind of indistinct darkness with a theme which features NOT death, as one could spontaneously think, but life (Langlais explained this very clearly in a letter to his classmate Henri Cabié). He expresses so a rather pessimistic feeling about the worldly life (see also the first part of Messiaen's Diptyque).
The death is featured by themes from the gregorian Mass for the Dead (a short citation on the trumpet at 1'15 and a longer one from 2'30). Around 3'10, we can hear, at the left hand under the trumpet, four notes from the gregorian Graduel (always the death) which will be repeated obsessively until the end with a characteristic "dotted" rythm.
At 3'35 a long crescendo begins ; the "dotted" rythm becomes more and more present : the death is progressively wining the battle and is more and more triomphant as the crescendo progresses.
BUT, suddenly at 4'55, the life theme makes his triumphant come back in E major with octaves on the big pedal reeds, under a kind of bells ringing at the hands : it's the resurrection, life has won.
The "life vs death" struggle was also treated by Messiaen in his "Combat de la mort et de la vie" from Les Corps glorieux and in his Dyptique.
Sorry for this long explanation, but perhaps it can help for listening to the piece.
I had to adapt slightly the last bars because they require a 60 notes keyboard and Aristide N°1 has only 56 notes, but tried to betray the original writing as little as possible. :-(