Snapshot wrote:The rules don't say that ones always fail.
And page 1 of the rules: "re-rolls happen before modifiers to the roll (if any) are applied".
^^This. There's no concept of natural dice rolls being special in
AoS.
But your broad thinking is correct that you modify the roll, not the required result, and this applies when determining other trigger effects too.
Another example, the Grey Wizard spell causes a -1 to hit penalty on a unit.
Forest Goblin Spiders hit on a 4+, but cause mortal wounds on a 6+. Cast the spell on the spider riders, they can no longer cause mortal wounds - no matter how many sixes they roll.
(Incidentally, this needed to go in the You Make Da Call forum as it's a rules query.)