Nightfall is a secondary effect to Shadow Strike. That’s to say, it’s like a devotion proc that triggers when Shadow Strike does so the damage it deals and ADCtHs off of are seperate to the base Shadow Strike node + Nidalla nodes. There’s a short guide to them here.
I’m not familiar with it but I have heard that the damage it deals get applied once per blade while dual-wielding so it should still be a nice amount of damage (like icing on the cake) and more importantly AoE to it. The AoE and extra hits in particular makes it a fantastic proccer for devotion skills that are harder to get going but important like Rumor.