From Dark and Darker Wiki
As far as I saw, it worked similar to the Barbarians Savage Roar ability with only low level enemies being affected by the frighten effect. If large monsters acted the same as the low level ones, it would make Bard a potentially OP must have for boss clearing, since the bosses would be unable to target friendly players for large periods of time.
A side note, I wonder how using Peacemaking would work with in conjunction with rangers for bosses? Could be a neat idea.