The world of the Thunder Age has a distinctly steampunk feel. The world has recently and drastically changed from medieval swords-and-sorcery to a mixed renaissance and Industrial Revolution tech level. Flintlocks and muskets are the best weapons an adventurer can hope for, but there are extremely expensive, very powerful machine guns called "storm cannons." The world has also recently fallen under the effects of the "Darkfall", an event causing thousands of demons or "nocturnals" to enter the world, along with Corrupted, those who have made a deal with the forces of the Darkfall for power-and usually a curse of some sort.
Source: Wikipedia, "World of Aden: Thunderscape", available under the CC-BY-SA License.