The Dread mask contains a psychic pickup which detects the worst fears of the wearer's opponent and amplifies them ten-fold, sending them running in terror. Any unit losing an assault against a Harlequin with a Dread mask must take their morale check on 1D6 more than usual (normally 3D6 instead of 2D6). Obviously the Dread mask has no effect against units which do not have to take morale checks.
Dread Mask
Source: Codex: Harlequins, p. 11