The Great Extinction

Eons ago, before written history, a great calamity impacted the planet. It is the greatest catastrophe to strike the surface since the planet's creation. It started with the great impacts. The asteroids mainly hit the sea floor, but their collision was still significant to the surface of the planet. The dust kicked up from the impacts clouded the air, making it hard for some species to see and breathe. It started with tsunamis and floods, then it caused tectonic plates to shift. The shifting created earthquakes and volcanic activity, which created some mountain formations along with the creation of new islands in what's now known as the Lapizan kingdom. The volcanic activity combined with the dust from the impacts only made the air quality worsen, creating thick layers of smog in most parts of the world. All of these changes happened within 100 years, only a single lifetime for the Petralumen species. Unfortunately, by the time it had settled down, there weren't many of the Unus Petralumen left. The remaining members of the species mutated and evolved to adapt to their new post-apocalyptic world. 
This great calamity is mentioned in some folktales and religious texts, but some still debate its legitimacy. It's been countless generations since the smog has cleared, far beyond the creation of the kingdoms, so it's natural that some people would be skeptical. There is, however, undeniable proof of these disasters in many parts of the world. The more recent islands of Lapiza are different in size and in shape from the older islands, providing the argument that they're far newer than the others. Skeletal remains have been found from a species similar to the Tribus Petralumen but with differences only explainable via evolution. A crater has been spotted under the surface of the sea, which has been thought to be one of the impact sites from the asteroids that hit the planet all those years ago.