And since it's a swift action, I'm assuming it can't happen on a confirmation roll (assuming swift action can't occur between attack roll and confirmation roll). True?
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Not sure of the legality of it, but I don't have a problem with the use. It would be limited to an initial lucky roll and then would only occur once per day. The only sticking point I might find is if an immediate action can be performed in the middle of an action occuring. I would say yes, as a movement that was interrupted by a pit trap would still allow a caster to pull off a feather fall. The immediate action would be mid movement action technically. The ability also mentions die roll, not action roll, a die roll could be occuring at any time during an existing action.