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To never have had before and to never have again. Often used to describe a unique achievement or occasion.
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在音乐史上，莫札特是空前绝后的音乐天才 | Zài yīnyuè shǐ shàng, Mòzhátèshì kōngqiánjuéhòu de yīnyuè tiāncái
In the history of music, Mozart was an unprecedented genius that we will never have again
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大家心目中的诸葛亮，是位空前绝后的军师 | Dàjiā xīnmù zhōng de Zhūgé Liàng, shì wèi kōngqiánjuéhòu de jūnshī
In everyone's mind, Zhuge Liang was the military advisor that the world had never before seen and never will see again
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This is an HSK chengyu
《宣和画谱》(Xuān Hé Huà Pǔ) by 赵佶 (Zhào Jí) of the Sòng Dynasty (宋, 960 - 1279)
There is technically no story behind this chengyu, as it comes from some lines from the work listed in the "Origin" section, but the lines themselves have a bit of backstory. To translate roughly, 赵佶 (Zhào Jí) writes that the world had never before seen a painter like 顾恺之 (Gù Kǎi Zhī), would never have another painter like 张僧繇 (Zhāng Sēng Yóu), and had never before seen and would never have again a painter like 吴道子 (Wú Dào Zǐ).
顾恺之 was a famous painter who lived during the Eastern Jìn Dynasty (东晋, 317 - 420), and he reportedly would draw exceptionally life-like people — but he refused to paint their eyes because he considered them windows to one's soul.
张僧繇 was a famous painter during the Northern and Southern Dynasties (南北朝 | Nán Běi Cháo | 420 - 589), and his style was exceptionally life-like too. According to one story, when he was hired to paint dragons on the wall of a temple, his work was so realistic that the creatures came to life and burst out of the temple. (You can read more about this event, which later became the chengyu 画龙点睛, here).
Finally, 吴道子 was a master painter and calligrapher from the Táng Dynasty (唐, 618 - 907), and like the two other painters, his work was very life-like. Once, he was hired to paint an image of the underworld on a temple wall, and anyone who set their gaze upon the sight reportedly abandoned their evil ways and turned to good.