
Spoilers
The incense is a very poisonous substance that the priests use to cause memory loss in Sha Na-in at the behest of Sha Amon.You can learn about why in the novel! The novel is better at getting across how all of Na-in’s attendants are uncomfortable with his lapses in memory, but can do nothing but play along. The slow build up of dread and horror about what’s truly going on and WHY is really good!

It’s because Nain’s mind is so fragile after living for so long and losing many loved ones, including his family, close servants, and even an adopted son. His melancholy also gets triggered by seeing humans die especially through sacrifice. When all of these sad emotions build up, he gets $u!c!dal. So that’s why Amon does these rituals intermittently- so that Nain’s mind can reset and forget those past traumas.
I’m just gonna add that Amon wants Nain to get used to human death by making him watch humans die. Yeah it’s screwed up lol but in his simple mind, he truly doesn’t understand human emotions. All he wants is to keep Nain alive. The novel does a beautiful job expressing his POV, why his thought process is the way it is, if anyone is interested
Can anyone of the novel readers tell me why the uke like passed out from the mist and had sex with seme unknowingly like a couple of chapters ago, I feel so impatient cus I wanna know so bad