The Lament of the Damned is an infamous battle formation first encountered among the forces of the Emperor's Children Traitor Legion. The Lament of the Damned is comprised of a terrifying onslaught of Chaos Sonic Dreadnoughts bound to the service of Slaanesh, the depraved Chaos God of dark pleasures.
Outfitted with the same destructive and mind-blasting arcane Dirge Casters and Sonic Weaponry as the Noise Marines, the Slaanesh Sonic Dreadnoughts that make up the Lament are shrouded in a howling discord.
This can drive their opponents mad and stagger the strongest of enemies merely by being within proximity to the Dreadnoughts, as reality itself rebels and begins to mutate under the power of their Warp-fueled assault.
This formation is made up of at least three Chaos Sonic Dreadnoughts and can contain as many as nine. The Dreadnoughts are led into combat by a fallen Chaos Lord or Chaos Champion of Slaanesh who has been eternally enslaved within a Dreadnought frame and daemonically-possessed through the enactment of occult rituals.
To be encased within a Chaos Dreadnought cybernetic sarcophagus is a curse of eternal damnation for a Chaos Space Marine, and doubly so for a worshiper of Slaanesh as it robs them of the pleasures of the flesh, driving them completely insane.
- Imperial Armour Apocalypse II, pg. 76