フラグが立たつ

expression, godan verb (-tsu)

a flag is set; a condition is triggered

Slang from computing/gaming: a variable or condition is set that will influence later events, often used metaphorically for story or relationship triggers.

• あのセリフで完かん全ぜんに死し亡ぼうフラグが立たった。

  That line totally set a death flag.