The term 'Grimm Opener' is a reference to the Brothers Grimm, two German authors who wrote a collection of fairy tales known as 'Grimm's Fairy Tales'. The two crossword solutions for this term are 'ONCE' and 'ONCEUPONATIME'. 'ONCE' is a four-letter word meaning 'at one time' or 'on a single occasion'. 'ONCEUPONATIME' is a thirteen-letter phrase that is often used to begin a story, especially a fairy tale. Together, these two solutions provide a fitting tribute to the Brothers Grimm and their famous collection of stories.