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lets say a unit of marines multi charges three separate units of seeker chariots. the marines target the riders and all together kill two alluress (no marines die) the third survives and loses combat and must now roll for demonic instability. would it be at a minus two from the wounds?? or at a minus four from all the hullpoints lost even though the chariots were not the target1??

You don't count hull points lost for combat resolution. You only count the number of glances and penetrating hits caused.

There are a couple things wrong with this question.
The first is that Chariots are vehicles and therefore do not take Instability checks. So in your example, the surviving chariot just stays there. No check is made, end of thread.

The other thing, CrownAxe answered. BRB under combat resolution foe vehicles tells you how many "wounds" a Glance or Penetrate are worth. Note that it doesn't say "hits" but rather Glances and Penetrates "suffered". So if any are saved via Invul save, they do not add to the "wounds" for combat res.
This is important to keep track of if a non-vehicle/ non-fearless unit loses combat in which a vehicle involved suffered Glances or Pens.

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