D is for Dungeons and Dragons and Demons

Dungeons
Although it is said that many citadels and fortresses of the Old Countries had underground complexes associated with them - and this method of construction has continued to the present day, these (usually) small subterranean complexes are often dwarfed by the massive partially-ruined underground compounds that can be found with disturbing regularity on the continent of Malaster. There are some whose original purposes can be discerned - whether they are the long-buried ruins of cities forgotten, with streets turned into corridors, and houses and shops into room complexes, dwarven mining complexes left to moulder by whatever disaster took the miners, tombs and funereal labyrinths, the dungeons and basements of fortresses long-since reduced to rubble, or something similar.

Many, however, have no coherent theme or purpose that can be discerned - some, indeed, seem to have only been the expression of some mad-god or magus' fevered imaginings, filled with deathtraps and puzzles, creatures that have no purpose beyond the assassination of intruders - sometimes literally - and treasures scattered haphazardly throughout (although whether they are purposely left by the creator, or lost by those who dared its hazards since, is often impossible to discern).

This plethora of underground (or partially underground) complexes has made exploration of the inner reaches of Malaster hazardous and a slow process - even in the earliest settled areas, there are still dungeons and other complexes found in areas long-since cleared out, or new sections of labyrinths long thought cleared are unearthed inadvertently. Some sages - although not necessarily well-regarded ones - opine that the entirety of Malaster's underground might be honeycombed with such complexes, natural and man-made. While this is given little credence by most, it certainly is the case that creatures thought restricted to one area or another of the continent are occasionally found hundreds or even thousands of miles from their native territories - with no plausible reason for why.

Dragons
Dragons are an exceedingly ancient species - some say they date back to when AO made all-that-is, while others say that the Draconic sub-species were, in fact, present at the Great Creation itself - although this is, naturally, madness, for before AO undertook the Creation there was only Chaos and AO.

The Metallic and Chromatic branches of the Draconic races are the most well-known, but by no means the only ones - by some accounts, there may be as many as a hundred different species of Dragon, ranging from the diminutive Faerie dragons, to the massive Reds and Golds. Dragons have changed - whether through accident of evolution, or purpose-driven change brought on by their own magics - to exploit nearly every possible niche, and are largely considered to be the apex predators in nearly every biome on Malaster.

Fortunately, they are exceedingly rare. Many - particularly in the more "civilized" areas of the continent - will go their entire lives without ever seeing* a dragon from closer than far in the sky (and grateful for that), and even in the wildest parts of Malaster dragon sitings are uncommon to be sure.

But those who have witnessed a dragon's attack - or those who have somehow survived one, or (more rarely yet) overcome a dragon - will never forget the experience, for even very young dragons are fearsome creatures. And the power of the most ancient and largest Dragons is rare and wondrous beyond belief.
*or, at least, without ever knowing they have seen a dragon, anyways, since many of the more powerful and cunning have mastered the secret of taking the form of others. 

Demons
Demons are those-who-disagreed-at-the-beginning - those who flouted the agreement with AO, and thought themselves unbound by AO's will. Although the term is often used by those with less knowledge of such things as a catch-all term for all such beings, Demon specifically refers to those who worked against the forces of Law in the Beginning, and were bound therefore to the Law in their very Chaos.

And so Demons, who are themselves Chaotic, are nonetheless held to be structured and rigid in their society and organization in the Places Beyond and the Outer Darkness. They, who created no beauty in the Beginning, themselves are believed to have none - even those who work through the temptations of the flesh are supposedly hideous in appearance, working through illusion and deception*.

Two things are certain, however - Demons can only enter all-that-is with the assistance of those already within, and if summoned with the proper rituals and circumscribed with the proper bindings and agreements, they are bound to serve the summoner, although such acts are, even when everything appears perfect, nonetheless risky to both body and soul.
*Demons of Temptation are widely held to be Erinyes, not Succubi, who are themselves held to be Devils by most sages - although as with many things "known" about the Outer Dark and its inhabitants, this could be the result of mischief, deception, or misinterpretation.