What is the DC for monster knowledge about dragons? And how many of those 50.000 dwarfs will make that DC, especially the higher level ones who rule the city/military?
This is my point. A lot of people go to a dice roll game mechanic to answer the question instead of going to common sense for a given campaign world.
If almost nobody in the entire campaign world knows that much about evil dragons (because evil dragons kill most humanoids that they meet), then there is no DC high enough to set for a DM. NPCs just do not know anything accurate about dragons (other than they are nasty, kill stuff, can fly, and have breath weapons, all things that can be noticed from a long presumably safe distance).
On the other hand, there would be a lot of inaccurate rumors about what dragons could do.
But you are thinking in game mechanics terms "What is the DC for monster knowledge about dragons?" instead of PC/NPC campaign knowledge terms "What rumors or information, accurate or inaccurate, exists in this campaign world about dragons?".
I think that part of this mindset came from the 4E designers who encouraged players having a ton of always accurate knowledge about the creatures that they fought. But the knowledge does not make sense. A bunch of wet behind the ears adventurers know all of this accurate information about monsters who typically kill whomever they encounter. But these same adventurers do not know any inaccurate information about these same creatures.
Did these adventurers go to monster school? How do they even recoginize the difference between a Goblin Thorn and a Goblin Sniper (both artillery with shortbows) such that they know that a Goblin Thorn can shift if it hits with a ranged attack and a hidden Goblin Sniper can remain hidden if it misses?
It's totally nonsensical.
In the real world, people have a general idea of how some things like cars and laptops and printing presses work because of education (and to a lesser degree the proliferation of the Internet), but ask someone details like have them repair a car or a laptop and most people are basically clueless. How much more clueless should a society be where very few people actually survive encounters with monsters, there is no level of education beyond basic skills and a single specific trade or profession, information tends to be local, and any information that is not local is spread from village to village by mostly merchants?
And that even assumes that the red dragon can shapechange as this is normally is quality of metallic dragons only and the preview did not show reds having that ability.
And again about the combat power of dragons, a single iron golem could kill the ancient dragon if the dwarfs manage to trap the dragon in a tunnel with it. So much for the power of ancient dragons.
Yeah, because every Dwarven city has a bunch of Iron Golems in their back pockets. Every trap that Dwarves set for Dragons are going to work and not be sussed out by an 18 Int creature. If a Dragon is trapped, it won't escape before an Iron Golem gets to the part of the city to attack it.
I would think that even in the unlikely scenario you describe, that the 18 Int 30 Str ancient dragon could grab the much smaller and weaker 3 Int 24 Str Iron Golem and use it as a weapon to break itself out of most types of traps. Sure, the golem could damage the dragon while the dragon is doing this, but the dragon could probably escape.
Alternatively, all the dragon technically has to do is have a good sized piece of ceiling drop on the golem, it then lies down on the rock. The golem is too weak to lift the rock and weight of the dragon off of itself and cannot use its breath weapon through cover.
Oh yeah, and Dwarves know all about Dragons with knowledge checks, but Dragons know nothing about Dwarves with knowledge checks.
I just find your scenario to be implausible. It makes more sense that an ancient dragon would kick the snot out of any dwarven city it came across and the dwarves wouldn't have the absolute best possible trap designed for a given type of ancient dragon.
