The story
Derived from a misquotation of a phrase used in the A Song of Ice and Fire fantasy novel series and its television adaptation Game of Thrones. In the novels, "and now his watch is ended" is used to conclude eulogies honouring fallen members of the Night's Watch, a brotherhood of men who swear lifelong vows to defend the Wall. "My watch is ended" is used in the episode "Oathbreaker" (2016) by a character who resigns from the Watch.
Etymology from Wiktionary, CC BY-SA 4.0.