Unprecedented response to rules questions from Games Workshop, the Wrath of Magnus FAQs are out and can be downloaded.
https://www.games-workshop.com/en-US/Rules-Errata?_requestid=1950227#40k-errata
Q: Do the units that are created count as Objective Secured if the Pink Horrors that spawned them had that rule? Are they a part of the Detachment that spawns them and therefore subject to all the rules of the Detachment?
A: No to both questions.
Q: Should the player pay points for the extra units formed when Pink or Blue Horrors split?
A: No.
Q: Do the new Pink Horrors get Malefic psychic powers?
A: No, they do not get Daemonology (Malefic) Powers.
Q: Does the entire spawned unit have to be within 6″ or just 1 model in it? Could I string a unit of Blue Horrors or Brimstone Horrors across the table, with just 1 model within 6″ of the Pink Horrors? It is not clear.
A: The entire squad needs to be within 6″.
Q: It seems that the only thing that stops them from Splitting is if they roll a Daemonic Instability test result that causes the entire unit to be taken off of the table. Is this correct?
A: Yes.
Q: What do you do if a unit of Blue Horrors or Brimstone Horrors cannot be placed because there are enemy models that prohibit this? I assume they are destroyed, or those that cannot be placed are destroyed, but it does not say.
A: Any models that cannot be placed are destroyed (the destruction of Blue Horrors in this manner does not create any Brimstone Horrors).
Q: If the entire unit of Pink Horrors is destroyed, do you place the Blue Horrors immediately? The Split special rule indicates that you only immediately place the Blue Horrors if a ‘rule’ causes the entire unit of Pink Horrors to be wiped out (which wounds like it is indicating something like Perils in the Warp, etc.) but what about simple attrition? If I shoot and destroy 10 of 10 Pink Horrors, how do I place a unit of 20 or 40 Blue Horrors within 6″ of the Pink Horrors if they are no longer there? Or, was the Split rule that talks about them being removed all at once due to a rule meant to indicate simply any time they are wiped out? It’s a bit ambiguous.
A: If a unit is wiped out, place the new unit immediately before removing the last model as a casualty.