That would of course depend on the base statistics. Obviously, you wouldn't what you have at first with 6+Int. mod. Also there are many ways to counter and avoid fear altogether. Personally, having it as a halmark while still being an average character in most other respects isn't broken at all.Cool idea, but I'm not a fan of class abilities that require a skill check. It's so easy to boost a skill into the ionosphere in 3e.
Would it be balanced?
I'm not sure i understand your meaning. Could you maybe demonstrate with another class?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.