It’s common to see on the battle field, armies with different detachments from different codex’s. Here I am going to go through why people do it and how it works. I will be focusing on the Death Guard and Nurgle daemon synergies as that is where I spend most of my gaming time, but it works the same with Khorne daemons and CSM Khorne units etc.
So anything Codex CSM, Death Guard or Thousand sons which has the daemon key word can benefit from a physic power or aura from a Codex Daemons.
For example if you have a Nurgle Poxbringer next to a Death Guard Plague Hauler, the Plague Hauler will benefit from the Poxbringer’s ‘Locus of Nurgle’ which gives all NURGLE DAEMONS +1 strength. This is because the Plague Hauler has both the NURGLE and DAEMON Key word.
Using the same method the Nurgle Poxbringer can cast the physic power ‘Fleshy Abundance’ on the Plague hauler because this has both the NURGLE and DAEMON Key word.
Games workshop ruled that you could not use the keyword ‘CHAOS’ as the common keyword when building a detachment. You can however use a particular God keyword such as ‘Nurgle’ or ‘Heretic Astates’
This is why you may see detachments in list like a:
Nurgle Spearhead Detachment
Poxbringer, 2 x Plague Haulers and Obliterators (mark of Nurgle)
These all have the ‘NURGLE’ keyword
Heretic Astates’ Super heavy Detachment
Magnus, Mortarion and a Lord of skulls.
These all have the ‘HERETIC ASTATES’ Key word
While this works and allows you to build lists of whatever you want within reason you won’t get all the benefits available.
If you keep each detachment from a single codex you then gain the benefits of the legion trait for example.
A Codex Daemon ‘legion trait’ is called a Loci and is different for each God. You only gain the Loci if the entire Codex demons detachment is made up of units from the same God eg Nurgle or Khorne.
The Nurgle Loci is ‘Locus of Virulence’ this gives all Characters from that detachment an additional aura, so any same God Daemon units within 6” of them do 1 additional damage on a wound role of 6+. This works in shooting and in combat.
Nurgle – if a Foetid bloat-drone with plaguespitters is within 6” of a Poxbringer he would have +1 to its strength and any wound roll of 6+ would be damage 2
Khorne – If a Maulerfiend was within 6” of a Blood master it would be +1 strength and be able to re-roll charge range
Slaanesh – If a Maulerfiend was within 6” of a Herald of Slaanesh it would be +1 strength and be able to charge even if it had advanced.
Tzeentch – If Magnus was within 6” of a Changecaster he would be + 1 strength and to turn hit rolls against him into failed results (this depends on the number rolled for the Locus of Trickery)
There is also access to stratagems to consider.
If you have a detachment which is made from one codex this gives you access to those stratagems and relics. If you mix codex’s within the same detachment using the ‘Nurgle’ key word for example you do not gain access to the stratagems.
Be warned ! GW did FAQ that you can’t use codex Daemon stratagems on daemon units from other codex’s.
Personally I don’t see any benefit in mixing detachments and not having the stratagems, unless you have gained access to the stratagems you want to use from other detachments.
It is all these extra layer of buffs why I think is really worth keep each detachment from a separate Codex.