All 3 Uses

solemn
in
The Masque of the Red Death
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But to the chamber which lies most westwardly of the seven, there are now none of the maskers who venture; for the night is waning away; and there flows a ruddier light through the blood-colored panes; and the blackness of the sable drapery appals; and to him whose foot falls upon the sable carpet, there comes from the near clock of ebony a muffled peal more solemnly emphatic than any which reaches their ears who indulge in the more remote gaieties of the other apartments.†

solemnly = with seriousness and dignity
with a slow and solemn movement
*

solemn = serious and possibly dignified
But from a certain nameless awe with which the mad assumptions of the mummer had inspired the whole party, there were found none who put forth hand to seize him; so that, unimpeded, he passed within a yard of the prince's person; and, while the vast assembly, as if with one impulse, shrank from the centres of the rooms to the walls, he made his way uninterruptedly, but with the same solemn and measured step which had distinguished him from the first, through the blue chamber to the purple—through the purple to the green—through the green to the orange—through this again to the white—and even thence to the violet, ere a decided movement had been made to arrest him.†

solemn = very serious--possibly dignified

Definitions:

(1) (solemn)
in a very serious (and often dignified) manner
(2) (meaning too rare to warrant focus)
Less commonly, solemn can mean that something was done with great or appropriate ceremony. It can also be used to describe something as dark or undecorated.