As long as the Plague Furnace has at least one wound remaining, the Plague Priest and the unit of Plague Monks pushing the Plague Furnace are Unbreakable, although neither can voluntarily leave the Plague Furnace.
Source: Warhammer Fantasy: 8th Edition
As long as the Plague Furnace has at least one wound remaining, the Plague Priest and the unit of Plague Monks pushing the Plague Furnace are Unbreakable, although neither can voluntarily leave the Plague Furnace.