Once a part of an angelic being known as Iutia, Desire was splintered off in Iutia’s dying breath, along with Reason and Spirit, each with a sole purpose - guarding their respective key fragments. While this was once the sole purpose of this construct, it's personality has been twisted by its want for what others have. At this point, the creature protects its key fragment out of spite more than anything else - everyone else wants it, so it will maintain it due to that fact.
Desire is a slightly convoluted fight, and focuses mostly on the actions of the players. Essentially the raw definition of a twisted desire, the construct yearns after every action it witnesses...and performs them in return. Desire retains perfect recollection of any and all actions, reactions and bonus actions performed since the end of its last turn. It can then utilize those actions with its Quick Mind action and Total Recall legendary action. So, for example, if your monk happened to launch a bout of blows quickly(like in a flurry or something similar), suddenly Desire can also do so as a bonus action for this turn.
Past that, Desire simply moves and attacks with Oeri. This fight has a bit of a ticking clock element, the damage from Oeri's festering wounds will quickly get out of control if the fight goes on for some time.
Desire doesn't have much of a personality. It was created with a purpose, but now is simply fueled by the want for what others want. BUT, it doesn't have to be a flat, two-dimensional creature. What kind of personality do you think Desire should have?
Once a part of an angelic being known as Iutia, Desire was split off along with two similar constructs, Reason and Spirit. A key was split into thirds, and each construct entrusted with protecting a fragment of it. It's said that not even the constructs know what this key is hiding. How much treasure could be hidden. Whatever is being protected, it must be worth it.
You could tease Desire by describing campfire tall-tales of a "behemoth" taking everyone's belongings. It simply shows up, takes what it wants, and leaves, slaying any who speak up against it. Perhaps there's way too many of these tales to be made up. Perhaps they even start hearing heavy footfalls right afterwards...
Interactions with Desire outside of combat will likely be few and far between, however should this happen, an oppressive, mysterious track should do the trick. Kell Rising from Destiny: The Taken King hits this very well.
Once combat hits, a tense, mysterious track will is perfect. Go For It from the Witcher 3 is stellar for this, having a slower start that mirrors the ramp up of the fight's intensity!
