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In the new Codex Tervigon, Venomthrope and Hive Tyrant with old-adversary all have rules that say:
all .... within 6 inches benefit from ....
So my question is must they be within 6 inches at the start of the turn, after movement or when they have assaulted for instance.
If it is after assualt then the Tervigon and/or Venomthrope and/or Hive Tyrant also all have to be withing 6 to 8 inches of the assaulted troops.
I interpret it to mean within six inches at the time they make their attacks. That's the only thing that makes sense to me, anyway.
It would need to be within 6" at the time of the attack, else you would see wording similar to "all within 6" gain preferred enemy for the remainder of the turn."
Notice that is does say units within 6" (Tyrant and Tervigon entry) and not units completely within 6" meaning that you only need a single model from the unit close enough to give the benefit to the entire unit.
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Same with the Venomthrope. Meaning you could have a string of Hormagaunts or Termagants all the way across the board yet still benefiting from the Venomthropes cover save. Slightly cheesy but thus far legal.