Salonen writes of the work that:
Nyx is my return to the genre of pure orchestral music since Helix (2005). It employs a large orchestra, and has exposed concertante parts for solo clarinet and the horn section.
Rather than utilizing the principle of continuous variation of material, as is the case mostly in my recent music, Nyx behaves rather differently. Its themes and ideas essentially keep their properties throughout the piece while the environment surrounding them keeps changing constantly. Mere whispers grow into roar; an intimate line of the solo clarinet becomes a slowly breathing broad melody of tutti strings at the end of the 18-minute arch of Nyx.
I set myself a particular challenge when starting the composition process, something I hadn't done earlier: to write complex counterpoint for almost one hundred musicians playing tutti at full throttle without losing clarity of the different layers and lines; something that Strauss and Mahler so perfectly mastered. Not an easy task, but a fascinating one. I leave it to the listener to judge how well I succeeded.
Nyx is a shadowy figure in Greek mythology. At the very beginning of everything there's a big mass of dark stuff called Chaos, out of which comes Gaia or Ge, the Earth, who gives birth (spontaneously!) to Uranus, the starry heaven, and Pontus, the sea. Nyx (also sometimes known as Nox) is supposed to have been another child of Gaia, along with Erebus. The union of Nyx and Erebus produces Day.