When I started this a,b,c malarkey, I wrote up a list of what I was going to say on some of the letters, just to make sure that I'd get important areas covered, and wouldn't accidentally use up needed letters, such as a throw-away post about Eldritch Abominations instead of the important Enclave concept.
H was supposed to be about history. But as I've struggled for the last day and half to come up with something to write, I've realized that I'm not ready. Alexis over at The Tao of D&D said some good stuff about campaign world history in his post about Pressure. In short: there isn't really any reason to make history that won't impact the adventures played in the campaign world. If the history isn't going to impact the players, there's a good chance that they won't care. (He does say some other things, specifically about Historical Pressures, but the above it what I concentrate on.)
Of course, this is not to say that there shouldn't be history in the campaign. A lot of history will impact the players, even if obliquely. But most of the time, you don't need the great picture. A complete family tree of the imperial family probably won't have much interest. But knowing that an NPC is distantly related to the emperor through his grand-aunt Margaret might be important. And you don't need a family tree for that tidbit. Placing the dungeon in an ancient tomb from before the time of the empire, with cultural artifacts from the previous civilization might be interesting. But mapping every previous culture back to the start of the ages (which was one of the options I considered for this post) might be counterproductive.
Because I love fitting history into my adventures. And I love for things to stick logically together. But I also love to create the biggest bang of a story. So I might find that I want an ancient lizardman empire. But if I have already filled out the history of the world, there might not be room for a lizardman empire. And then I'd either have to scratch that idea, or find a way to tweak it.
Alternatively, I might have a vague idea of history. Various cultures and empire will be dotted along the time-line because I've mentioned them before, either in background (the creators of this or that artifact) or because they have been encountered in play (that ancient tomb from before the time of the empire). And the lizardman empire can be slotted in anywhere that there is room. They might even expand upon known history. I might have known that the Solomani were destroyed in a series of wars. Now the lizardmen could be the enemy that they fought, and lost, against.
I already have a few historical tidbits forming. I know that the peak of the empire was a bit over two hundred years ago, as I established that in the F is also for Firearms post. I know that various factions out in the empire has rebelled and seceded successfully since then. I'm pretty sure that the capital has had an unbroken string of emperors, and has never been touched (in large scale at least) by civil war, though a few barbarian invasions may have made their way to the walls. I'm also have an idea in the back of my head that this empire isn't the first, and that the remains of earlier capitals can be found beneath the city. But honestly, I have nothing worth writing a post about yet.