The word "decree" originates in the early 14th century from Old French decre (or decret), dēc stemming from the Latin dēcrētum ("ordinance, decision"), which is the neuter past participle of dēcernere,  meaning to decide determine, or separate.  It entered Middle English to describe official authority-based orders and originally meant sifting through evidence to make a decision.  It was initially used in ecclesiastical (church) contexts, "decree" evolved into secular usage for legal orders and authoritative announcements by the late 14th century.  

According to the Oxford English Dictionary (OED), the earliest known use of the noun decree in English is from 1303, found in the writings of the poet and historian Robert Mannyng.  The verb decree appeared in the late 14th century (roughly 1399).  The OED also notes the related term, decreet (noun), first appeared around 1374 in the works of Geoffrey Chaucer.  

A decree in storytelling acts as a powerful narrative device that establishes authority, drives plot, defines character relationships, and imposes structural order on a narrative world. It is a formal, authoritative command or decision—often issued by a king, deity, or governing power—that serves as an inescapable law within the context of the story.