What's the nightfall challenge thing I keep seeing being mentioned? Is it another modifier like the heroic/hard mode?
The nightfall strikes have multiple modifiers to make it more difficult which are dependant on the strike, you can view what each of these modifiers are by hovering over the icons for them in the lobby after selecting the mission. The modifier that makes nightfall strikes a lot more challenging is that if the team wipes, they will need to start from the beginning and not the last checkpoint.