Originally Posted by Reginald the 18th
What would they best work to count as though? Is there any particular unit that suits this thematic vein? Tzeench seems the most fluff-favoured, but Pink Horrors don't really fit the style. Daemonettes fit little better statwise, but does necromancy really suit Slaanesh? Are there any other daemons units I'm forgetting? Ideally fast, melee focussed types.
If your theme is necromancy and you're using different models, really you can count them as however you want. The faction keywording may be a bit tricky at first but if you run more than one detachment you should be able to fit in more than one alignment of daemons. Don't worry about how well the actual daemons fit your theme, just decide what kind of ghosts you want and choose the most apt rules.