Hmm...

dungeon = dýflissa, dyblissa, dybliza, myrkvastofa

But in D&D "dungeon" usually doesn't mean "dark prison"
but something like "huge underground complex". I don't
think we have an ON word for that.

The 'ormr' is fine for "serpent". For a more specific
"dragon" you can try 'dreki'.

Kveðja,
Haukur