noun which may take the genitive case particle 'no'
worst ... of one's life
Used as a no-adjective to describe the worst experience, event, or thing in someone's life. Often appears in phrases like 人生最悪の日 (the worst day of one's life).
あの日は人生最悪の一日だった。
That day was the worst day of my life.
人生最悪の失敗をしてしまった。
I made the worst mistake of my life.
最悪 means 'worst' in general, while 人生最悪 specifically means 'the worst in one's life'.
Compound of 人生 (life) and 最悪 (worst).