Unheard-of; unprecedented—an extraordinary event or situation with no prior example.
前代未聞の大雨で町が浸水した。
The town was flooded by unprecedented heavy rain.
そんな前代未聞の事件は歴史に残るだろう。
Such an unprecedented incident will go down in history.
前代未聞の高額入札が話題になった。
The unheard-of high bid became a topic of conversation.