Taking any other kind of detachment as a child detachment (Chaos Daemons, Death Guard, Thousand Sons)
Taking any HQ choice (except Fabius Bile)
This triggers a validation error stating that the HQ unit must have a selection of "Supreme Creation", but that option is not (and should not be) visible and cannot be added.
Removing the child detachment doesn't make the error go away, but if you then remove and re-add the HQ choice, the error doesn't reoccur.
This triggers a validation error stating that the HQ unit must have a selection of "Supreme Creation", but that option is not (and should not be) visible and cannot be added.
Removing the child detachment doesn't make the error go away, but if you then remove and re-add the HQ choice, the error doesn't reoccur.