Cassandra rushes over to the door leading into the great hall. It's sturdily made and designed to prevent fumes from leaking out, but there's no disguising the noises and rumbles she hears.

Spoiler: Cassandra
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You hear the thuds and scraping of a huge creature moving around on the stone floor, followed by a rumbling yet high pitched voice with a hollow echo:
"Someone's behind that door."

Draconic Frightful presence, Wis DC 19 or frightened for 1 minute, repeating the save at the end of each turn.
Ad hoc advantage on your save since you can't see it.
(1d20)[11] or (1d20)[10] + modifiers

Notes:
You don't have languages filled in on your sheet, but wizards normally speak Draconic so whether she's speaking in Common or Draconic, you'd understand it, and dragon voices reverberate and carry.
Frightful presence is, in combat statblocks, something that the dragon "uses" for free every round, but it's not really something the dragon switches on or off... it's just always on because it's a big terrifying dragon.