One of the arcing backstories that may or may not affect campaigns set in the World of Onn are two major cataclysms that have affected major empires and even life itself with a near E.L.E. (Extinction Level Event).
Onn's deep past before the rise of Man and the Demi-human races featured a lizardkin race at the height of supremecy in magic, knowledge and power. Extremely rare artifacts (mundane and magical) have been found by various adventuring parties over the years in my games, but there was never any connection made by the players. Simply, the objects didn't have anything to do with the plot or adventuring premise, but if the players wanted to open some additional venues of exploration, they could have. The Lizardkin were enshrouded in mystery due to the Age from which they lived and unless some truly extraordinary means were used by the players, not much information could be gained. Near the end of the lizardkin's civilization the humanoids rose up from creation and learned advanced knowledge from the lizardkin. The Lizard Dynasty ended when they bent their magic to eradicate one of their own enemies, and the retaliation of said enemy's magic altered Onn itself and split up the supercontinent Onn-gaia in an exremely fast manner (not geologically speaking either). How the magic affected the lizardkin is unknown, but their entire civilization swiftly fell. This is known as the First Cataclysm by the few scholars that have gleaned the knowledge from the mists of time.
Onn's Second Cataclysm ended the Zuhn Empire's existance and nearly all life on Onn itself. Known simply as the "Fire in the Sky", in one instant much of the interior of the lands of Ossus were obliterated (centered directly on Zuhn's capitol city) and a line of destruction carved out the Bay of the Ancients, sundered 2 mountain ranges in half (the Godshield and Argentrock ranges), Bottomless Trench and created the Ironsands Desert. The heat and intense winds from the blast kicked up so much grime into Onn's atmosphere that most of the life that wasn't affected by the event died in the months after. Only the hardiest survived. Scholars lament the loss of knowledge the Zuhn collected that was obliterated. Some think a star fell from the heavens or a meteor, others some magical devastation of epic proportions. The cause is unknown, but there are areas of the Great Waste that people go into healthy and come out of sick, wasting away until they die. Creatures never before seen have been spotted as well, some resembling twisted man and scorpion hybirds, giant versions of common animals, etc. The effect of the devastation was powerful enough to rip the veil of the planes as well, causing destruction in the Faerie Realm and the immigration of Elves to Onn.
Even though Referees may not choose to include any of the historical elements in their campaigns of either Cataclysm, they do have an effect on play how some races view the world and other races. Elves, for example, are a fearie race new to Onn. Having only been residents for about 300 years, they still know little of Onn or its past, they are far from the wise, all-knowing people of most other campiagn worlds. Forged Men are an example of technological and magical oddity. Forged Men existed in the time of the Zuhn Empire (though PC Forged Men are only a few years old), what have they been doing for the thousands of years it took living creatures to rebound? Why aren't they the dominant life form? What secrets of the Zuhn do they still possess? Though the history of Onn should remain in its past, thoughtful Referees should at least give a thought or two on how this could affect their campaigns. Because the Zuhn had magical and technological power advanced farther than the current Age, historians, scholars, Kings and other power-hungry poeple are always searching for adventurers to search out this old ruin or delve into an old tomb purported to be the resting place of some important dead inventor. Most of the time this will be a bust (as far as the hiring party is concerned), but it will line the adventurers' pockets with good gold and their reputations.
Just food for thought.