Some units can stalk with ease through terrain that is treacherous for others.
In comparison to Epic 40,000, we derived this ability from part of the rules for Tyranid war engines. This allows for more flexibility in unit design.
Where your unit has the Ignore Terrain ability:
While you move the unit, you can have it cross dangerous terrain without need for a dangerous terrain test, or even cross impassable terrain.
[TODO: Should we qualify this to the effect that you can’t end a move on impassable terrain (probably best), or at least that you can’t end a chain of moves on impassable terrain?]