na-adjective
awesome; incredible; formidable
A fixed expression derived from classical Japanese (恐る + べし), used as a na-adjective to express admiration, awe, or sometimes ironic amazement at something impressive or daunting.
彼の集中力は恐るべしだ。
His concentration is incredible.
このゲームの難易度は恐るべし。
The difficulty of this game is formidable.