Others have touched on this, but to clarify, you do not need DLC to play Nightfalls [i]unless[/i] the featured strike was originally added to the game with an expansion, in which case, you need the respective DLC to access that particular Nightfall strike. If the current week's Nightfall is a strike that was introduced with The Witch Queen, then you need to own The Witch Queen to play it. If it's a strike that's been in the game since launch, then you do not need any DLC. It has always been this way.