YOU SAID:

Their prowess is potent and secretly stern.

INTO JAPANESE

その力は強力で密かに厳しいです。

BACK INTO ENGLISH

That power is powerful and tough secretly.

INTO JAPANESE

その力は強力でタフな密かに。

BACK INTO ENGLISH

Its power is strong and tough secretly.

INTO JAPANESE

その力は強く、タフな密かに。

BACK INTO ENGLISH

Its power is strong and tough secretly.