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FAQ:
Q. When a model with a Plague Censer causes a
wound on a ridden monster or other model with
separate target-able components, how is the
damage distributed?
A. Take a Toughness test with the best Toughness
value available to the ridden
monster/chariot/multiple statline model and then if
the test is failed, randomise the wound as you
would for a missile hit.
As for the Hellpit, I would suggest (without reading the rules) it inflicts 1 hit distributed as shooting.