The Wailing Doom is the weapon borne by the Avatar of Khaine, the burning iron incarnation of the Aeldari war god that wakes upon their craftworlds only in times of dire need. Whether it takes the form of a spear or a sword shifts with the moment, but its nature never changes: it is molten wrath given edge, a thing that shrieks as it strikes and leaves ruin wherever it falls.
When a craftworld's Avatar is roused, warrior-sacrifices are made and the iron god rises from its shrine, cracked skin glowing with the fury of Khaine's death. In its grip the Wailing Doom cleaves through armour and flesh with equal ease, and its cut carries the searing heat of the forge that birthed the god. Warlords of the ancient enemies of the Aeldari have been split asunder by a single blow, their defences meaningless against a weapon that is part relic and part divine agony.
The wailing that gives the weapon its name is said to be the voice of Khaine himself, forever raging at his sundering in the War in Heaven. Each swing renews that lament, a mourning cry that is also a promise of death. To the Aeldari it is both terror and hope, for the Avatar walks only when their need is greatest, and the Wailing Doom answers that need in fire, in slaughter, and in the shrieking memory of a god who will not die quietly.