Units in base contact with one or more Fiends of Slaanesh suffer a -1 penalty to both Weapon Skill and Initiative. This has no effect on Daemons of Slaanesh.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
Units in base contact with one or more Fiends of Slaanesh suffer a -1 penalty to both Weapon Skill and Initiative. This has no effect on Daemons of Slaanesh.