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If I'm understanding correctly if you include demons in a chaos army then you can't have a legion trait. I know for sure the demons don't benefit from the trait, but I'm not 100% that it stops everyone else from taking a trait. Most codexes specifically state as such, like IG codex specifying which units disrupt the ability to take a trait and which dont. This is made stranger by the ability to summon demons and demons being in the chaos space marine codex.

So can I take demons in my chaos list and still choose a legion trait for the chaos marines in the army.

Yes, but they'll need to be in different detachments.
As per page 156 you only get the trait for units in Chaos Space Marine Detachments, which are defined as detachments including only models with a <LEGION> or equivalent keyword. As Daemons don't have that, mixing them in a detachment breaks the legion trait. But note this is only at the detachment level. Having a CSM detachment and a Daemons detachment is A-OK.

Yes, if you include daemons in your CSM detachment, you can't benefit from a Legion trait.

Note that if you summon daemons, they're not in a detachment and therefore don't affect you when it comes to determining if you qualify for a Legion trait.

 Ghaz wrote:
Yes, if you include daemons in your CSM detachment, you can't benefit from a Legion trait.

Note that if you summon daemons, they're not in a detachment and therefore don't affect you when it comes to determining if you qualify for a Legion trait.

This is the correct and most complete answer.