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I believe this works.

A CSM character can join a Khorne Daemonkin unit b/c they aren't a seperate mark?

A Chaos demon character can join a KD unit but not vice versa, correct?

Chaos Daemons units and characters with Daemonic Instability cannot join or be joined by any units or characters that do not have Daemonic Instability, so they cannot join any KD units.

Chaos Space Marines characters that are undivided or carry the Mark of Khorne can join KD units, but they cannot join or be joined if they both have different marks.

Pythius Primus wrote:
Chaos Daemons units and characters with Daemonic Instability cannot join or be joined by any units or characters that do not have Daemonic Instability, so they cannot join any KD units.

Chaos Space Marines characters that are undivided or carry the Mark of Khorne can join KD units, but they cannot join or be joined if they both have different marks.

Maybe I'm reading too far into this, but as stated in the BRB:

A unit with this special rule cannot be joined by a model without this special rule.

SO for example:

Khorne herald from Demons could join a unit of KD Flesh hounds??

In this case the IC is joining the unit, and not the other way around.

Essentially:

So a model with this special rule is joining a unit without this special rule

You are misunderstanding the rule. Quoting from the codex:

Daemonic Instability: Cannot join (or be joined by) models without this special rule; automatically pass Fear, Pinning and Morale tests.

Under no circumstances can anything with Daemonic Instability join or be joined by something that doesn't.

Pythius Primus wrote:
You are misunderstanding the rule, possibly because you are only reading the summary from the back of the codex rather than the full Daemonic Instability rule.

Under no circumstances can anything with Daemonic Instability join or be joined by something that doesn't.

Not sure I can link here, but trying anyway.

This says that a unit with this special rule can't be joined by a model with this special rule.

In this case we have the opposite:

A model with this special rule is joining a unit without this special rule

Its not clear in the main rules, but if you look in the summary, its clarified a lot more clearly . You cannot join or be joined.

Captyn_Bob wrote:
Its not clear in the main rules, but if you look in the summary, its clarified a lot more clearly . You cannot join or be joined.

Precisely:

Don't you love how Game Workshops summery of a Rule may contain additional or different instructions to the Rule itself?

Or that the "Designers Note" is the only way to know how to bring KD summoned units onto the board?