If your set on the Warlords and army in general revolving around nightfight.
then I'd drop the "choose" and 3+ roll for determining nightfight from the list and either give them a special rule or piece of wargear that either allows you to choose which turn nightfight is on, or re-roll the dice for determining nightfight depending on points.
that way the army isn't rellying on a D6 roll to do what it is supposed to do.\
infact, make cry of the nighthaunter a relic that allows the bearer to re-roll the dice when determining nightfighting (either way) and drop the points a little.
would be a good way of letting the nightlords "choose" the battlefield, against a superior assualt army that is immune to fear (nids) it would let you try and engage them during the day, thus ensuring they did not get hte benefit of nightfight. But against an army like tau it would ensure you live long enough to get into a position to hammer their lines.