“ Dust Elemental
https://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=1594
At the corner of air and earth elemental planes resides the dust Elemental, a growing threat to Parisia. With it's powerful choking attack, this creature is a threat to the city. More and more of them are appearing in town, spawned from the many dust devils, a frequent foe of the party, they come in size from 3 HD to 20 HD. Why are there so many? Why in the city?”
“ After the PCs defeat (or force the rapid retreat of) a villanous necromancer/demon summoner, they discover a book. This book outlines how to summon a demon whose power increases according to the size of the summoning circle used to summon it. And after the PCs examine a map of the country, they find that the layout of the cities and roads match up with the required summoning circle. In fact, the final road that would complete the circle is currently in construction.
With anyone who can find out about the circle and the ritual to summon the demon able to somehow make the final road/line, and destruction of the cities and roads currently in existence out of the question (unless this is an evil campaign- that removes the moral quandary), how are the PCs going to solve this problem?”
“ The PCs find a small green glass bottle, something can be seen writhing within the smoked glass. If the PCs open the bottle a repulsing form of life will pour forth and grow in size. A genderless beings of soft pliable purple flesh towers before the PCs, it has no detail, no distinguishing characteristics except for a narrow slit for a mouth that holds rows of inwardly curving teeth.”