nidan verb (upper class) with 'ru' ending (archaic), yodan verb with 'ru' ending (archaic), nidan verb (lower class) with 'ru' ending (archaic), intransitive verb
Archaic verb form; in modern Japanese, 恐れる is used instead. Found in classical literature and fixed expressions like 恐るべき (fearful, formidable). Multiple archaic conjugation patterns (Nidan/Yodan) and marks it are listed as intransitive.
古典では「敵を恐る」のように使われた。
In classical texts, it was used like 'to fear the enemy'.
「恐るべき」は現代でも「恐るべき才能」のように使われる。
The form 恐るべき is still used today, as in 'formidable talent'.