This monstrosity of a bomber is something that everyone should avoid if they don't have some serious anti-air weaponry. The Harbinger first saw action while besieging Cadia during the 13th Black Crusade. Usually dropped off high echelon-bases, aircraft carriers or other craft capable of fighter transportation directly at enemy forces. Of course, being a super-heavy means that manoeuvrability is a problem here, thus Harbingers are always escorted by Hell Blades and Talons.
The scary part about the Harbinger is that despite its limited manoeuvrability, the damn thing makes it up with some serious amounts of dakka in the form of four Twin-Linked Autocannons (two nose mounted and one on each side of the craft) and if that's not enough, then the thing can take forty eight bombs (that's like four twelves, and that's terrible). Most of the time it's incendiary bombs, but the bomber can also swap them for the even more rapetastic Hellstorm Bombs, nuclear payloads, or Nurgle flavoured Blight Bombs.
Most of the time, Harbingers are piloted by four hardwired Servitors, but it can also be piloted by Daemons (via possession), or by a single pilot or two (normal cultists or Chaos Marines). These last ones vary in skill from complete amateurs to ace pilots (or even double ace pilots).
To finish this, the Harbinger can be also outfitted with Warp Rift technology that results in the sky blackening and causing severe storms that give enemy aircraft turbulence and the probability to get shot down by Warp Lightning.