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In a matched play game units in a detachment must share at least one common faction keyword, and that cant be CHAOS. When you mix CSM and daemons in a detachment the common faction keyword is CHAOS. Units (including belakor) gain the DISCIPLES OF BE'LAKOR keyword, but thats not a faction keyword.

2. MUSTER ARMIES
All of the units in each Detachment in your Battle-forged army must have at least one Faction keyword in common, and this keyword cannot be CHAOS, IMPERIUM, AELDARI, YNNARI or TYRANIDS, unless the Detachment in question is a Fortification Network (this has no effect on your Army Faction).

https://wahapedia.ru/wh40k9ed/factions/chaos-daemons/slaanesh#Army-of-Renown:-Disciples-of-Be-lakor

No, DoB is legal in matched play. You just need to keep your Chaos Daemons and CSM in separate detachments.

As above. Just don't mix within a detachment.

Looks like thats the way to go, you need two separate detachments, so there is a CP penalty if you want to play the disciples of belakor.

Don’t all DOB units have the Disciples of Belakor keyword, which allows you to mix them in a detachment?

 alextroy wrote:
Don’t all DOB units have the Disciples of Belakor keyword, which allows you to mix them in a detachment?

The issue is that its not a faction keyword.

 Stux wrote:

 alextroy wrote:
Don’t all DOB units have the Disciples of Belakor keyword, which allows you to mix them in a detachment?

The issue is that its not a faction keyword.

RAW this is correct.

However, RAI I'm pretty sure the initial design thought WAS to allow mix'n'match daemons and CSM in one detachment. The restrictions are onerous.

I'm curious if this'll get FAQ'ed.

In any event, keeping them in separate detachment as others said for matched play is the only legal build.