...it makes more sense here than a fierce gown. What is a fierce gown?
I understand you: "a fierce gown" is not any thing in nature, but, however, the phrase "to be clothed in a fierce gown" (which, after all, is verbal rather than nominal), though it employs very oblique and poetic language, is easily seen as meaning "to be clothed in a gown which makes one appear fierce", does it not? This is quite common in the business world: the jackets of men's suits traditionally are padded in the shoulder. Why? To make the man wearing the suit appear more masculine/effective/commanding... in a word: fierce. This is simply an example of a man employing "the trappings of power."(For, as character Cersei Lannister mentioned in the novel
A Dance with Dragons: "It (is) never wise for a ruler to eschew the trappings of power, for power itself flows in no small measure from such trappings." - I personally found that bit of prose to be quite profound for a novelist such as George Martin, more worthy of such a writer as Robert Greene...I tore the page from the book (it was a paperback), and set it aside) By that token, and based upon that rationale, saying that "she was clothed in a fierce gown" appears tantamount to saying: "she appeared fierce in the gown which adorned her", or more expansively: "she was dressed in a gown obviously intended to make her appear fierce and effective to onlookers".
Whoa!!! See...fierce! (and something else as well...)