If you die at a level transition while facing away from the loading zone, you will fall into it and trigger it while dying, which places you on the other side at negative health while still alive. In this state, most enemies ignore you unless you attack them. If your health ever goes back above zero, enemies will attack you again, and the Winter Lanterns are, so far, the only enemy that can see through the glitch.
I initially discovered this while grinding the troll/crows at the entrance to cleric beast for blood vials and blood echos after defeating cleric beast, though at first I just thought I was surviving the attack with so little health left it looked like I was dead. Then after a few more times of getting greedy when fighting that specific troll, I realized that I definitely should not have survived, so I tested and figured out that you can get your health to go negative enough that a single blood vial will not get you back to positive health, and then eventually found out the invisibility portion of the glitch (unfortunately not until AFTER I had already triggered the Darker Night, making it significantly less useful. Would've been nice for exploring early on).