Q (Big Flag): Shaitan, the original owner of the cursed arm that is the True Assassin’s Noble
Phantasm “ Delusional Heartbeat” is said to be an “evil spirit” or “demon god” or “demonic
man” depending on the literature. What category does he fall under?
N: I believe “evil spirit” would be the most correct nuance. An incarnation of a spirit that was tainted
by the human world. However, the incarnation was after his fall, so most of his authority is already
gone. It’s rough that I can’t just simply say that he’s the devil.
T: Why can’t you say so?
N: Think about each country’s characteristics.
T: Ooh.....

Heaven's Feel I Booklet Nasu & Takeuchi Q&A