Both Uses

formidable
in
Throne of Glass
(Auto-generated)

Compared to the sunlit hallway, the interior that stretched beyond them seemed formidably dark, but as she stepped inside, candelabras came into view, along with black-and-white marble floors, large mahogany tables with red velvet chairs, a slumbering fire, mezzanines, bridges, ladders, railings, and then books—books and books and books.†
p. 54.7

formidably = in an intimidating or impressive manner
They made a formidable pair, gods help them all.†
p. 166.1 *

formidable = intimidating or impressive

Definitions:

(1) (formidable)
intimidating or impressive -- arousing fear or admiration due to impressiveness or challenge
(2) (meaning too rare to warrant focus)