It depends. Do you want the piece that they reference the most, or the most comprehensive answer? The short answer is Guardian from the D1 soundtrack, aka the original title screen music. They reference this track all over D2 like there's no tomorrow. The long answer is Music of the Spheres. Before Destiny entered production, Pete Parsons asked Marty if he could write music for Destiny ahead of time. Marty created a suite of eight pieces called "Music of the Spheres." So the answer is that there is not one Destiny theme, there are eight. All eight pieces of Music of the Spheres make up what are supposed to be the Destiny main theme. Music of the Spheres was completed almost 2 years before the game came out and was supposed to be referenced and edited/adapted over the entirety of Destiny's lifespan. You can listen to it here - https://www.youtube.com/playlist?list=PLDOdDCOrPnmk4cZpScZ-ZJ8Xt5WgdkuAc (Only the first 8 videos are part of Music of the Spheres.) Since you said you're a musician, I think you'll enjoy this: the music was called a "musical prequel." Basically each of the 8 pieces was meant to be a chapter in the overall MotS story about what happens before the base game. Think of it as Halo: The Fall of Reach, except instead it's a symphony and not a book. Each piece is based on a planet in the ancient Greek concept Musica Universalis, or the belief that the planets revolving around the Earth (geocentric model) produced a note or hum, and the note they produced effected the planet's attributes and how they interacted with each other. Each piece in MotS is based on a planet. For example, track 7 (The Prison) is about Saturn. In Musica Universalis, Saturn is the planet of "the major misfortune," or "Infortuna Major" and also the planet of death, loss, etc. So The Prison is a very scary, unsettling piece to listen to. The Moon is supposed to be the sphere of feminism, beauty, slowness, and legato motion, so The Path is very slow and beautiful (among my favorite pieces ever). Each piece told a story about the Destiny storyline as well. So Mars is the sphere of war, so the Mars piece (The Rose) is about the battle against the darkness. At the end of The Rose, the Traveler sacrifices itself. The Ecstasy, the next piece, is a royal funeral for the Traveler. The Ecstasy is based on Jupiter, aka the king of spheres. The inspiration for the piece being a funeral comes from Jupiter's real world red spot, which the composers thought would be a wound of some kind. The piece starts with a massive, kingly ceremonial fanfare and leads into a hymn about the Traveler. I've already typed a lot and yet I haven't even gotten into the Geomancy stuff or the Golden Record stuff, or even Alpha Lupi or the poetry. Music of the Spheres has a set of 14 poems written for it by a Cambridge scholar named Malcolm Guite. You can find those on the internet too. Sadly, all of this is massively unknown by most Destiny fans. You can ask most people if they know about MotS, and they'd say no. Even those who do know about MotS don't know that Bungie commissioned poetry for it or the other stuff. Last thing, Marty assigned each piece to be centered around a note in the Lydian flat-7 scale. There are eight pieces, so the eight key signatures for each piece go in order: C, D, E, F#, G, A, Bb, C Hopefully this was helpful. The dance is as deep as you wish it to be.