The Rictus mask projects an aura of death, filling those nearby with an almost uncontrollable urge for morbid self-preservation ("We're all doomed!"). Any enemy unit with a model within 6" and line of sight of the model with the Rictus mask must take a Leadership test at the start of their turn. If the test is failed the unit may do nothing that turn. If the unit is fighting in close combat then the more immediate threat of being diced overcomes their horror and so units that are in an assault are not affected by a Rictus mask. Models that are immune to morale checks are unaffected by a Rictus Mask.
Rictus Mask
Source: Codex: Harlequins, p. 12