The mod isn't quite finished, but I'm simply splitting with the lore.
The lore is massive, so it is spoilered.
We all know how it is today. Dwarves trading with humans, angering elves, stabbing goblins, humans angering elves, goblins stabbing humans, goblins angering elves, and so on. It hasn't always been like this. You see, like individual worlds are split into Ages, a higher history is split into Eras. Regular games happen somewhere near the center of The Third Era. Before I go on to explain what caused unexpected changes, you need some background...
Eras are distinct segments of purposely-recorded history. The time before then is regarded as The Zeroth Era.
During the Zeroth Era, not much truly happened. Dwarves developed in the caves, humans developed in the forests, as did the elves. Not much is known about the goblin's history; their first recorded appearance was near the end of The First Era. The First Era begun once dwarves developed the first pick. How exactly it was made is unknown, but of what little was recorded, it is assumed it was made out of bone; presumably crundle bone or something similar.
Once dwarves pierced the surface, and once they got over the fact a ball of magma was taped to a glass dome above them, they met humans. The fledgling humans met the dwarves in a forest; the humans taught dwarves how to not vomit from being in the sun, and the dwarves taught the humans how to make booze, how to kill trees, and how to strike the earth. The good relations formed stay with us to this day, although humans seemed to have learned how to con before dwarves, as shown by their sales of picks. After a while spent getting inebriated, the dwarves continued on, into the part of the forest they hadn't used for barrels.
In the deepest parts, they found the elves. Due to their striking similarities to humans, the dwarves assumed it was some sort of camp, and offered them barrels made of wood full of beer. The elves were horrified by the dead tree products and tried to attack - the single miner in the group killed every attacker, as is to be expected. This is why elves don't attack immediately after being robbed or shown a "rude bauble", although they still send suicidal gangs of ill-equipped rascals to give the dwarves free clothing.
After all these introductions, the dwarves became very tired of the day and night cycle, and tried to return to the caverns. Realizing that they were hopelessly lost, they took a trip to the largest, rockiest object in sight; a large mountain. The band of 7 dwarves made the first mountainhome; they eventually opened the caverns and enlightened the rest of the dwarves on the great things they saw, the people they met, and the elves. It has become dwarven tradition to send a ragtag bunch of seven dwarves to uncharted areas to carve out a new home for their species.
After the first introductions, not much happened during The First Era. Some important inventions were made, such as the still, which was made after a fortress fell due to food not fermenting fast enough, the door and drawbridge, initially designed as "anti-elf protocols", and the well, after it was discovered that alcohol interferes with the dwarven healing process for major wounds. While the first dwarves to claim this were outed as elven spies, it was eventually realized that, despite how crazy it sounded, it was true.
The Second Era began after an event with no name occurred; this was before creative naming of events was invented. From the horrifying plains in which not even the craziest of dwarves wandered, goblins first walked. Fossil records show that they had apparently been around for some time, but were heavily uncivilized until near the end of The First Era. Goblins first met the humans, where they met moderate resistance for little reward. They next met the elves, who they crushed with enough ease it was decided that they would be ignored by most major attacks, since there was no fun in it. Finally, they came across the first mountainhome, fortified with the vomit of a thousand woodcutters. Here, they were greeted with large, full-scale attacks by steel-clad maniacs; they saw enough of the dwarven riches that they decided it was worth every loss. For every dwarven fortress that falls, thousands succeed at surviving goblin sieges. From things learned from the first sieges, goblins built the first dark fortress. Its completion marks the end of The First Era.
The Second Era, much like The First Era, had few truly significant events. Most notable are that humans taught dwarves how to use beds after a tragic Giant Cave Spider accident that took out half a military, six farmers, three nobles, and a miner. Some scholars say that dwarves would be far more advanced if beds weren't introduced. These same scholars are hard to come by anymore, as they suffered similar fates, most notably a fatal Fluffy Wambler ambush that any conscious dwarf of any age would have survived. Other innovations from The Second Era include the crossbow, the warhammer, the two siege engines, and the domestication of the wagon.
The beginning of the most well-known era is in dispute, but, between Urist McHauler the 57th's 104th birthday, the 50,000,000th goblin siege on a dwarven settlement, and elves becoming slightly less stupid, it is a difficult choice. Let's review them all, just for the sake of fairness.
Urist McHauler the 57th was a hauler, born during The Late Second Era, from Urist McHauler the 56th and Urist McLegendaryIdler the 124th. During his life, he hauled many things, from gypsum, to cat leather, to various bone crafts. He even once had the glory of hauling a Forgotten Beast's corpse. He married Urist McDabblingEngineer the 11th when he was 58 and she was 57, and they had two childre, Urist McHauler the 58th and Litast McKilledByWolves. While the 58th McHauler had kids of her own, Litast McKilledByWolves was killed by wolves at the young age of 31. His 104th birthday was significant since the last McHauler to live to be that old was the 26th, and Urist McGreedyNoble the 267th decreed that all further McHaulers would die tragically funny deaths before reaching 104. How exactly he was correct for 31 generations was unknown, although given the lower age-of-death in that fortress than others, it might have simply been a statement.
The 50,000,000th goblin siege on a dwarven settlement was against the sprawling fortress of Bluedaggers, which had fought off 99 other sieges of its own. This siege consisted of exactly 100 goblins and 100 trolls; 10 goblins were weaponmasters of some sort. While Bluedaggers won handily, secretly, everybody knew it was the 50,000,000th siege.
The final, least reasonable possibility, was that elves had a renaissance. Exactly what would've happened that could have caused one, or what one could involve, is disputed, but fossil records show far more exotic animals in former elven settlements after a certain point. Either way, these events had something to do with the beginning of The Third Era.
The Third Era was a time of great development, with dwarves using fairly simple technology to make unusual contraptions, including things described as "computation devices". Compared to the dwarves, everybody else pretty much stagnated. However, one major event at the end of The Third Era was the most influential era-ending event known.
It is called... The Mood Period. During The Mood Period, dwarves had discovered exactly how to make strange moods happen, how to choose what job it is relevant to, and even what materials. This was also a time of massive child creation, with dwarven populations increasing tens to hundreds of times faster than average, leading to more settlements. At some point, someone discovered how to make people who had moods before have another, leading to a second wave of great artifacts. In the rush, entire settlements were drained of materials, causing dwarves to invade elven forests for their subterranean bounties and the wood they lived in. In their rush, they also conquered human towns, dismantling everything and everyone; many fell moods were had, but the invasions reduced fell mood-related dwarven deaths. During the surge of dwarven strength, the ones who developed the technology were killed through unknown means; like most humans, their fate is assumed to be by the hands of a fell dwarf. Once moods were left to chance again, everything stabilized, except for the fact nearly everything was dead, and goblins barely made it out alive, having a couple pits up in a terrifying savanna. Armok, who had minimal interaction up until now, was forced to interfere, and reconstruct the world.
Dwarf Fortress: Rebuilt is a sort of reconstruction of Dwarf Fortress. Some things in the raws are carried over, but most are being reinvented.
The base for it is being constructed, but is almost complete, with only a few item folders, a few necessary plant types, and most animal types remaining.
(I really hope unreleased mods can be posted here - it should be released in a week or so)