I've posted my explanation for the Scouring but in case you want a more detailed version for the "History of Ortho" it boils down to this...
*****LONGGGGG EXPLANATION****
The Mage Wars were just fought by the Provinces of the united OCA against Heka. Basically, this is just one of those stupid wars that happen in real world history. Heka's ruler had managed to tick off a substantial number of the Harmonium's Founding Races despite being a Founder of the KOH. Voll was interested in expanding its wealth and power that they more or less re-drew the map so that they would have dominion over Heka with the Full Support of the Orcs/Beholders/Hobgoblins (seriously; the biggest problem in the Harmonium is that so many of it's leading races are EVIL).
The Mage Wars were more or less the wizards of Heka being crushed under the heels of the united OCA and the entire province put under military occupation. The Gnomes are more or less restricted to Heka (I don't explain this but you can elsewhere) because they've been enslaved by the Wizard Lords of the Pre-KOH Heka for as long as can be remembered. Furthermore, the Gnomes seem to have been native to Heka. They were freed by the Knights of Harmony when Heka was liberated from it's former Lich King but have no real power base (being freed slaves).
The Gnomes start keeping their money and slowly build up their fortunes.
Enter Flayn the Fourth. Basically, he's a human supremecist and would-be Alexander the Great who wants a glorious war to prove himself. Also, he's insane but this is being covered up by the Voll since the Great Grandson of King Rohmel being a raving Nero-style nutter wouldn't really do for the public. Unfortunately, unlike with the current Flayn, they're not smart enough to take him out of power or replace him with a double like they did in the modern era. Flayn is approached by Akashi the Destroyer who provides him with the chance to prove himself.
Akashi arranges a vast number of terrorist attacks that point to the Gnomes and a general Gnomish uprising with them framed as "Soldiers of Chaos." Meanwhile, the OCA is dealing with all the usual **** that a government is dealing with in local rebellions plus setting up mass transit or whatever they're doing after only 70 or so years in existence. They more or less give Flayn a bag of money and tell him to handle the Heka situation as he sees fit (As Akashi knew they would---it's implied that Akashi is behind lot of these problems)
Under Akahi's advisement, Flayn buckles down under his disgust and hires a Hobgoblin Army. This is because Flayn's own troops won't be inclined to conduct the kind of ethnic cleansing that Flayn has in mind. The Hobgoblins are almost totally soldiers and the chance for a large amount of war booty gained by annihilating the Gnomes is promised to them. The actual Hobgoblin government has nothing to do with Flayn's hiring them and probably thinks that it's all perfectly legitimate mercenary work (again---Hobgoblins wouldn't really stop to think that this is unusual).
Being Hobgoblins and only six or so generations from Acheron, they more or less begin the Pogrom of the Gnomes and the destruction of their cities with only the slighest shrug of their shoulders. It's from a legitimate Ortho authority, they're getting paid, no one really gives a crap about Gnomish lives to begin with. Meanwhile, Flayn has moved all of his own Voll forces away from the massacre to guard the human settlements in the region (he can do that since he's King and been given authority to handle things by the OCA).
Remember, Hobgoblins obey orders and do their job quietly. The Orcs would be laughing and bragging across the planet.
Flayn, of course, would have been found out and probably executed by the OCA if things had gone on. You can't really hide the genocide of an entire species and if he'd dispatched his own soldiers against the Hobgoblins, he'd still bear the blame for hiring them. Flayn IV, being mad, doesn't care and considers wiping out the Gnomes to be a great victory. I imagine his circle of advisors helping him carry this out are scum as well (they WOULD be getting a lot of loot from the newly rich Gnomes). Honestly, a lot of Gnomes were probably in hiding or still alive though when things make all of Flayn's plans irrelevant.
Akashi, at the mere price of leading another race to annihilation, has actually been using the genocide as fuel for a spell that releases The Burning Death. It's a gigantic flaming conflaguration that covers the two kingdoms. Frankly; it wipes out a lot of the human towns, finishes off the Gnomes, annihilates the Hobgoblin armies, and probably slaughters countless Voll as well. Flayn IV is grabbed bodilly and soul by Alzrius to the Abyss (or he simply died in the blazes----either story works).
Akashi gets his revenge on the homeland of two of the Knights of Harmony. Bluntly, it's probably the single greatest disaster the OCA ever suffers with the exception of Juhein's Civil War and Nemausus.
(Remember, Akashi is immortal so killing him isn't something the Harmonium doesn't try in retribution for this).
It takes a very long time to pick through the ashes but, eventually, both kingdoms rebuild with Voll having endured the best of it and more or less resuming the rulership of Heka. The Harmonium takes a couple of decades to figure out what the Hell happened. Frankly, the Voll and Hobgoblins point fingers at the other as to who is responsible for the Gnomish genocide (most probably assumed they died out in the fire initially). Mostly, the Hobgoblins correctly point out they had nothing to do with it and all of their members responsible were killed in the blaze.
In truth, the Hobgoblins as a whole don't give a ****. The Hobgoblins find the idea of wiping out the Gnomes more or less the equivalent of pest control but they don't share that view because it's unpopular (even if the Orcs and Beholders would be quietly nodding their heads if they did say that). Also, most of them are probably *****ed they never got paid and so many Hobgoblins died because of human stupidity.
So, any objections?