In Final Fantasy VIII, Bahamut has a little speech:
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.
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.