I’ll keep this post short and simple. The iPhone X camera array has an IR illuminator. IR stands for infrared, it sees in a different spectrum and is used for almost everything night vision. Yes, FaceID will work in the dark, IR all but guarantees that. Along with the IR illuminator, the screen lights up when you try to unlock it, so your face is illuminated by the screen and the IR illuminator confirms you are a physical 3-dimensional person. In short, again, yes, FaceID will work in the dark.