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Is it possible to take the Mayhem Pack or any variant there of in a Khorne Daemonkin army list and have them with the Blood For the Blood God! rule? Or would you have to take them in a Vanilla CSM formation detachment? I'd love to take those insane bastards in the formation and get the blood tithe points when they pop.

No. Its a CSM formation

Pretty sure you could take one like you can any formation... (or am i missing something?)

they just dont get the DK special rules.

No. Khorne Daemonkin are a different faction to Chaos Space Marines.

The fact that both can field Helbrutes is no different to the face that there are several codices which can field "Dreadnoughts" - but Dark Angels, Blood Angels, Grey Knights and regular Space Marines can't 'trade' formations.

Ye sure?

'
Allied Formations
Formations do not count as your army’s Allied Detachment, even if they are made up of units from a different
codex to your Primary Detachment, and they do not stop you from taking an Allied Detachment in the same
army. However, the Levels of Alliance rules from the Warhammer 40,000 rulebook do apply to them and units
chosen from a different codex that are in the same army.
For example, if you included an Ork Formation in the same army as a Primary Detachment from Codex: Space
Marines, then the units from the two Detachments would treat each other as desperate allies. However, the Ork
Formation would not stop you taking an Allied Detachment in the same army.
'

from the helbrute dataslate. If you can't freely swap in and out any army's formations why does it give such an act (all be it an extreme one) as an example.

Allies rules allow you to put any formation or detachment in your army. But, they will deal with each other as the allies matrix and rules dictate.
You can have a KDK and a CSM formation in the army, and anything else you want. But, they might react badly to each other.
The quoted section then says you can take a Primary and an Allied, and other stuff too.

all be it < albeit

I think you misunderstand each other.

Yes, a mayhem pack can be taken along KDK armies, but it will use CSM helbrute rules, not KDK ones.