A strong or impressive beginning followed by a weak or disappointing ending; an anticlimax (four-character idiom).
プロジェクトは当初は注目を集めたが、資金不足で竜頭蛇尾に終わった。
The project initially attracted attention, but due to lack of funds it ended in an anticlimax.
彼のスピーチは導入が良かったが、その後が続かず竜頭蛇尾になってしまった。
His speech had a good introduction, but it didn't follow through and ended weakly.
小説を書くときは、読者に竜頭蛇尾だと感じさせないように全体の構成を考える必要がある。
When writing a novel, you need to plan the overall structure so readers don't feel it's an anticlimax.