By Alexandra Desplat and played in DH part 2.

The main theme in DH Part 2, and one of my favorites.

From the Harry Potter Wikia: "Lily's Theme" is a subtle choral piece, related to Lily, Harry, and Snape. It is the main theme of the Deathly Hallows: Part 2 soundtrack.

The full theme can be heard in the song of the same name. Besides that, it can be heard on the tracks Dragon Flight, Voldemort's End, Snape's Demise and The Resurrection Stone.

"Lily's Theme" begins with a dark sounding note, then a chilling and haunting choir begins, then there's a sudden crescendo and decrescendo of cello, viola and violin. The choir then continues and the theme ends. The theme is then repeated without a choir, just instruments. It slowly builds up in volume, reaches its pinnacle, then its goes down in volume, with the theme ending. It then goes down to pianissimo and stops.