All 3 Uses

solemn
in
Nation, by Terry Pratchett
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"He's right, Mau," said Milo solemnly.†
Chpt 6. *

solemnly = with seriousness and dignity
She saw the big man stretch himself out with a cracking of joints before turning to the little crowd and saying, in the slow and solemn voice of a man who tests the truth of every word before letting it go: "If anyone touches the stones, they answer to me."†
Chpt 8.

solemn = very serious--possibly dignified
Once she solemnly handed her baby to Daphne, and Cahle made a remark that Daphne didn't quite catch but which made all the women laugh, so it was almost certainly something like, "It's about time you made one!"†
Chpt 12.

solemnly = with seriousness and dignity

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.