A Harlequin's mask incorates [sic?] a holographic generator that projects images of monstrous faces and demonic visages, intensified by a short-range psychic field which increases their opponent's sensitivity to fear and despair. Any enemy unit fighting in an assault against a Harlequin reduces their Leadership by -1.
Mask of Fear
Source: Codex: Harlequins, p. 6