The synonymous words of great momentum were: great fanfare, magnificent mountains and rivers, all-powerful wind and clouds, swallowing mountains and rivers, vast seas, ten thousand horses galloping, magnificent, heroes achieving success, and majestic.
The words 'don't mention it' were 'don't talk about it, don't mention it, don't mention it, keep your mouth shut, hide it in the bottom of your heart, remain calm, and keep your mouth shut.
Synonym fiction refers to fictional works that play with synonyms. It might involve using many different words with similar meanings to create a rich and nuanced language in the story. For example, instead of just using the word 'happy', the author could use 'joyful', 'elated', 'gleeful' etc. throughout the text to add variety and depth to the description of a character's mood.