Summary: A cruel empire used famine to annihilate its enemies, but is forced to desperate lengths to ensure its own survival. They succeed in a way different than they originally envisioned.
The elven empire, with all its iniquities and atrocities, rose over their mastery of nature and agriculture. Compared to the human nations of their home continent, their lifelong experiences in forestry and farming gave them literal centuries of advancement. Human historians of the era attributed the elves' low populations, but apparent lack of farms, to druidic magic or fey powers. The truth was far less germane.
The elves were even more fertile than humans, but few of their children survived their first month. Elven children were inspected, and left to die of exposure or hungry creatures in the wilderness. Those that survived were raised in brutal schools, where competitive combat training and blood sport were routine parts of the curriculum. After their first century, they were as dangerous as only a handful of their human foes.
The elves were not immortal, although they had longer biological lives than humans. Despite elven historians' boasts, humans actually adopted hygiene and sewer systems before they did. They had another way to cheat death, by means of a peculiar procedure. Scholars would come to understand how it worked in later centuries, imbibing a cultured moss to mimic patterns of neurons, including 'copies' of memory and personality. The elites of the elven empire, thus, passed themselves into the bodies of the young, merging the vigor of youth with the cynical cruelty of the wicked aged. Even their cultivation of the Scythian womb-trees to duplicate themselves did little to change their low populations, more of paranoia than pragmatism.
The elven armies rarely marched, although not for lack of bloodlust or concern for nature. Elven armies avoided pitched battles, releasing crop blights, scorched earth tactics, and invasive species to devour food stores. As human city-states and tribes withered behind their fortifications, diseases spread among the besieged peoples. Wherever the elves marched, starvation and plague followed. Subject human populations were enslaved, their populations carefully controlled for the empire's needs.
As with all empires, they reached their limits. While the elven fleets reigned almost unchallenged upon the high seas, their slave-armies ruled upon the land, and their coffers overflowed with stored food and gold, they became complacent. Their aristocrats indulged in diets of excess, eating themselves to obesity and then starving themselves, or regurgitating the food of a large banquet as a competition between aristocrats. When an elven governor did this in a starving human city, it set off an uprising.
The elven military had to redoubled their garrisons, which left them exposed. The most paranoid of the aristocrats, seeing the very real possibility of the rebellion's success, engaged in their own contingency. In ghastly experiments upon innocents, they tried fusing the Scythian womb-trees with the cultured spores. As their empire burnt, they vanished into their dark, dank keeps. The elves, thus, found a method approaching immortality: a strain of womb-tree able to grow adult bodies, containing their own memories, transmitted by means of the cultured spores. Such plants could forever ensure their survival, the aristocrats reasoned, through their proliferation.
The elven aristocrats departed on private expeditions, to remote and unknown lands, while others went among the newly-liberated people as collaborators. The human rebellion, already war-weary, was unable to pursue the elven remnants. Their leaders changed to other priorities, and in time, they came to make war upon each other and new threats. Unlike the elves, the humans embraced new ideas, and in time, came to exceed their former masters in most respects.
In time, human explorers found the elven womb-trees, scattered to the far corners of the world. Some were thriving colonies, prosperous city-states or ornate villages. Others were bogs or tangles of infected flesh, whose swift termination was a merciful euthanasia. The humans had not forgotten the elves, nor the specific leaders that had escaped justice. Some humans even adopted similar procedures to the elven elite, but such techniques were not widely accessible at the time. In time, certain human nations inflicted their own revenge upon the elves.
The womb-trees were hard, if not outright impossible, to annihilate completely. So the humans turned to their spawn. From the trees came a crop of elven clones, identical in likeness and personality to the specific forebears that seeded it. The humans quarantined them, reducing them to a squalor akin to their own subjectivity. The elves, who lived through periods of overeating and excess, now knew true hunger. Their struggles only just began.
The elves that died of lingering starvation or wasting disease passed the least painfully. Each year, a new crop of clones emerged, despite the older ones' best efforts. Left isolated near the trees, elven ruthlessness once more arose. The older clones, those surviving a few seasons, organized an annual cull of the newest tree-born, whose remains were cannibalized. In theory, the elves may have been able to organize resistance against their captors, but they cared for themselves first, far more than even narcissistic humans.
From the outside, the human survivors of the elven empire observed grimly. The elves devoured each other, sometimes even physical and mental clones of their younger selves. In time, the elven prisons became charnel houses of cannibals. The elves thus became ghouls, eager to feast upon the flesh of the living and dead. Even after their empire collapsed, the elven elites' mastery of nature and agriculture dominated their destiny.