Basically, this has to do with a glitch in the E4 rooms that allows you to enter a ton of areas that can normally only be entered by hacking/events. In fact, you can skip the entire E4 and just go to the end.
Well, Kabigon found a way to get to the area with Darkrai using this glitch. Only downside is, if you try to catch it, you will get stuck to unable to get out without restarting... and if you save, say good-bye to your savefile. Anyone who wants to try to find a nonlethal method to using this glitch is welcome to try, but be aware of the risks X)