The vehicle is covered in potent scriptures, purity seals, icons of faith and inscriptions of abjuration. It is extremely difficult for a Daemon or its kin to approach such a potent symbol of faith. Any Daemons attempting to assault the vehicle must pass a Difficult Terrain test to reach it, and suffer -1 to their Leadership when in base contact with or when they are tank shocked by the vehicle in question, in addition to other modifiers.
Sacred Hull
Source: Codex: Daemon Hunters, p. 19