How Do You Lay Siege To A Dragon?

This is adventure 18 of 25 that is roughly designed to take characters from level 1 to 25 that I will be sharing over the month of September. These posts are a part of Asshat Paladin's OSR Short Adventure Challenge, and utilize his Get Ready, Get Set, Go! format.

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How Do You Lay Siege To A Dragon?
A Get Ready, Get Set, Go! Classic D+D adventure set in and around The Big Generic City

Get Ready:
Having chased the dragon across the wilderness, it has arrived in The Big Generic City, and has taken up residence in the castle in the center of town.

Get Set:
In addition to the new draconic overlord, the town is also dealing with constant violent storms since the dragon’s arrival, and they’re getting worse. The king is marching his army to the town to attempt to deal with the creature.

Go!:
The King is perfectly willing to sacrifice The City and the army he sent if it’ll save the rest of his domain. The army includes Exeter, a lieutenant of the King who has been instructed to negotiate with the dragon.

Magical energies are fluctuating (1-2 less powerful, 3-5 normal, 6-9 more powerful, 10 more powerful + backlash, roll once per round).

If the PC's didn't go to war with the hobgoblin army, they may be available to assist.  However the PCs decide to deal with the dragon, remind them that there are still lots of terrified civilians in the city and that the city may not be the best location to deal with the dragon once and for all.

Important NPCs:
Exeter (F14, King’s signet ring, broach of protection from dragon’s breath, sword+5/+8 vs Dragons)
Frenzied Demonic Dragon (AC -6, HD22, Attacks Ground: 2 claw/1bite (4d4, 4d4/6d6+6) Flying: Swoop, Crush, or Hover, Spells (5, 5, 5, 4, 3), Demonfire Breath - 180’x30’(current hp damage), Morale 10) – The Dragon will flee the city if reduced to half her hit points.