from the death_flag wiki:
Pixiv Tag: 死亡フラグ (Shibou Furagu)
A sign to the audience that a character is about to die, and/or otherwise foreshadowing an imminent bad end.
A death flag is usually raised by a character saying or doing something that can be seen as tempting fate. Examples include: Doing something that he or she has been warned, sometimes repeatedly, could be fatal; an unrequited relationship suddenly and finally being requited, especially if it comes out of the blue; a promise to return just before leaving on a journey of uncertain length; and so on.
See also pool #1246.
Seems pretty subjective to me, and covered by pool #1246 already. Is this tag needed?
Updated by TheKid965