Models assaulting into, out of or through difficult terrain will be slowed down in the same manner as models moving through it in the movement phase. Roll two D6 and pick the highest to see how far they can move.
Terrain Effects
Source: Core Rulebook 3E, p. 62
Models assaulting into, out of, or through difficult terrain roll two D6 and take the highest for their move distance, as in the movement phase.