The story
Coined by American writer and television producer George R. R. Martin for use in his A Song of Ice and Fire fantasy novel series. The phrase valar morghūlis, which means all men must die in the fictional language of High Valyrian, is used as a greeting by the Faceless Men, a combination of a death cult and assassin's guild. The traditional response is valar dohaeris (“all men must serve”).
Etymology from Wiktionary, CC BY-SA 4.0.