The Culexus' special attacks do affect daemons that are psykers. This includes units of daemons. Blasts from the Animus Speculum, the effects of the Life Drain ability and any blows struck by the Culexus Assassin in close combat count as psychic attacks and daemons receive no saving throw on behalf of their daemonic aura.
Daemons of Khorne are not affected as they are not psykers.
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