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So Myself and some friends were discussing some rulings from the new Traitor legion book and how to use them when we came across this situation that we're somewhat divided amongst.
The Night Lords "In midnight clad" formation rule states that Night Lords can force night fighting in a game and that while night fighting is in effect, they have +1 to their cover save, meaning they have a 5+ cover save in the open.
However, if we pit them against Tau battle suits, they have a black sun filter that allow them to ignore the penalties for night fighting.

The conflict we have is that the night fighting penalty is stealth for the attacked unit which Night lords have inherently, not the +1 to their cover save, so would the night lords still get a 5+ cover save despite black sun filters or would the +1 to their cover save be ignored instead because it is granted by Night fighting, despite not being a direct penalty of the night fighting instance?

What is the exact wording of the darksun filter?

You have permission to ignore stealth from night fighting. You do not have permission to ignore the bonus from night fighting due to nocturnal warfare.
The fact that NL have stealth inherently makes me think it does nothing

Black sun filters gives you night vision. So you'd ignore the unit gaining stealth but would not ignore any other bonus. As night fighting is in effect. So they would still get the plus 1. Also it would only ignore the stealth given by night fight and if the unit has stealth already they'd still keep it