Bloodcraft: Bad Omen
Following the deaths of many illagers at the hands of an enemy, their wrathful spirits are said to converge into a beast of vengeance. This event is considered a myth in Overworld, though one old book describes a battle won by Piglins against two illager clans, and it was a short lived celebration. A horror appeared and tore them to pieces.
One survived and he shakily told of a mass of dark energy or some kind of physical monster that couldn't be felled by any weapon or spell.
This entity will not form if illagers kill other illagers, only when other sapient races do so. It is also said a parent's grief can trigger the formation and possibly cause possession of the mourning illager; giving them the power to slaughter those responsible for their child's death.