Iron Daemons ignore terrain classified as Obstacles and do not suffer D6 wounds for moving through Dangerous Terrain, except Rivers and Marshland, which are treated as Impassable terrain.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
Iron Daemons ignore terrain classified as Obstacles and do not suffer D6 wounds for moving through Dangerous Terrain, except Rivers and Marshland, which are treated as Impassable terrain.