At a prestigious university of a thriving multicultural mega-metropolis, a team of top alchemist/scientists discover the impossible - a small portal that produces any element you can think of: gold, water, diamonds, etc. and it seems that the artifact steals the requested materials from afar. It is not a divine artifact, and in fact, has a psionic anti-divine/blasphemous aura of some sort.
The scientists have a heated argument: we must have it for the betterment of humanity, no, this belongs to the gods, no, we must take this to the elves because the elves funded this research project, no, the elves will just kill us all and keep it their secret, no wait, this artifact may be tainted and must be studied carefully before further decisions are made, no, we should use it immediately, no, this thing is wrong, everytime we use it to gain wealth and resources someone else will lose their wealth and resources, etc.
With inflamed passion and greed, there is an accident, anger to leads to another few deaths, and finally the human scientist is standing in the lab amongst his slain non-human colleagues. He takes the portal and rushes off to the human section of the palace, which, of course, goes into an uproar. Those who advise cooperation and diplomacy and caution are drowned out by the fear and greed of the majority. The eventual consensus is that the metropolis with its advanced arcane/scientific edge is poised to rule the lands in combination with the unlimited wealth and resources promised by the artifact. Nobody can be trusted, of course, because everyone (including the gods themselves) will try to wrest this device from our hands. The metropolis might even be wiped off the map by wrathful deities or covetous demons. Yes, offense is the best defense, and we will rapidly expand into an empire, terrible and awesome to behold, become so mighty that no one can despoil us. We shall learn the secrets of the universe so that we fend off the jealous gods themselves. Contracts may have to be drawn with neutral entities (perhaps the devils, after all, we'll have unlimited gold to pay them) to give us an early advantage and early defense. And so forth.
I also like the plague idea if it's meant to be a more gradual process, although, I assume the campaign has to account for a plague of such proportions.
I regret the deus ex machina of this artifact (it seems almost every D&D story starts with an artifact), but I feel this serves as both the drive and advantage that propels the metropolis into an effective (ie, non-suicidal) xenophobic scientific-revolutionary empire, and I guess it could be plunked into a standard campaign setting as is more or less.