The "Death Eaters theme" is a theme associated with the Death Eaters. It is a dark, brooding piece that gradually builds up to a crescendo. It first appears in Half Blood Prince Soundtrack.
Full list of appearances
the Death Eaters theme can be heard during the opening, The Burrow Attack and when Malfoy lets the Death Eaters into Hogwarts. It is played by a string, brass and percussion ensemble. Can be heard on the tracks "Opening", "Into the Rushes", and "Malfoy's Mission".