grammar - What form is 恐るる?

In Final Fantasy VIII, Bahamut has a little speech:

…G.F.とは我らのことか
我らを力として使うとは…
恐るるべきは人間どもよ

I'd never seen two るs doubled up like that. Which conjugation is this? It seems to have plenty of Google hits so it's not a typo, as far as I can tell.

Answer

A short answer: the form 恐るる (おそるる) is the attributive form (連体形; れんたいけい) of the verb 恐る (おそる) in classical Japanese.

A long answer is as follows.

Classical Japanese has different conjugation rules from modern Japanese. The verb 恐れる (おそれる; to fear) in modern Japanese was 恐る (おそる) in classical Japanese, and its attributive form was 恐るる (おそるる).

If I understand your example correctly, the speaker is a mythological creature, and mixing classical Japanese in its words is probably meant to show that the speaker is very, very old.

However, 恐るるべき is actually incorrect even in classical Japanese. The correct conjugation in classical Japanese is 恐るべき. I do not know why the writer of the video game used the grammatically incorrect 恐るるべき instead of 恐るべき, but here are some speculations.

1. Although 恐るべき follows the classical grammar, it is in the usual vocabulary in modern Japanese and is not an archaic word. The corresponding phrase in the modern Japanese grammar would be 恐れるべき, but I guess that 恐るべき is much more common than 恐れるべき today. It is possible that the writer of the game chose the form 恐るるべき because it sounded differently from the common expression today and it emphasized that the speaker was archaic.


2. 恐るるに足らず (“not worth fearing”) is another common fixed phrase which follows the classical grammar, which would be 恐れるに足らない in modern Japanese. It is possible that because of this phrase, the writer incorrectly assumed that 恐れる in modern Japanese should be always translated to 恐るる in classical Japanese.