Welcome to your normal European style fantasy setting: where mana is a self-renewing natural-resource, where biodiversity has given logistics the old one-two punch, and unlike in modern settings, the gods chose to enact a more 'hands on approach' rather than relying on their prophets to speak their voice.
This is also by definition a setting where humanoid like races, (humans, and their cousins the halflings, drwarves, elves, gnomes, etc) are the threatened species, with heroes and adventurers given the lucrative, and fatal, occupation of keeping monster populations in check for whom said humanoids are very much their natural prey.
Unicorns, nymphs, fairies, dryads, satyrs, pixies, brownies, and the sort act as nature conservationists.
Some Unspeakable Things buried insanely deep underground for good measure...
And naturally there were dragons (the western, interested in wealth and power kind, not the benevolent and divine eastern kind) ... and ogres (or Oni as they were sometimes called).
And this is where things get interesting.
Dragons and Ogres both had a sub-species that they had bred through magic for centuries to become the ideal servant race. The kobolds for the dragons, and the goblins for the ogres. They'd been engineered to where blind unquestioning devotion to their masters had been genetically ingrained in them!
But this isn't quiet where things start off for our story.
The humanoids and the fae put their own difference aside long enough to provoke a war between the the dragons and the ogres, leaving both empires devastated beyond recovery, and their already natural small numbers reduced even more so, along with any genetic diversity. And their population of goblins and kobolds obliterated. And then the humanoids and fae struck to finish off what was left.
Only a fraction of the original dragons and ogres remained, hiding in their crumbling fortresses, without their labor force to sustain them, and no longer having the infrastructure nor overwhelming advantage over the 'lesser creatures.' Going from able to make entire nations quake, to now being a non-entity among the world's power players.
Now that we have that unnecessarily elaborate back story out of the way, We can get to what you came here for. Sort of.
Both goblins and kobolds feared for their extinction, and that of their masters, fearing no longer being there to serve them, or them no longer being there to serve. So they'd made precautions.
The cursed gems had been placed where they wouldn't be easy to find, but far from impossible, and in fact, where hidden in such a way, that they'd do nothing entice people to find them. Stories scattered of some 'great treasure' with conflicting descriptions, all to lure in the curious or the greedy.
It was rather ironic, or maybe fitting, that the kobolds and goblins had both come up with the same scheme for preserving their kind. And as fate's way of keeping the great game entertaining, both cursed gems were found around the same time.
A human and an elf found the goblin cursed gem, buried (but not deeply) in a cave juuuust out of the way enough, with the clues so obvious that only the stupid or brain damaged wouldn't be able to solve them.
"My praise to whoever wrote these clues, they're so elusive and perplexing!" Said the blond elf with her male human companion.
"I know what you mean! But the treasure is finally ours!"
The two dug up the green glowing gem... the moment their eyes laid on the gem, their eyes turned the color gold... and their skin became rubbery and green... their height shrank, but their bodies compressed, becoming squat and stocky.
The female elf's body's figure became exaggerated even as she became shorter. Her companion's male features likewise became exaggerated, growing bigger. Their clothes dissolved off them, their feet like the soles of boots and their rubbery skin like leather. Their ears grew larger and (more pointy in the elf's case). Their noises bigger, their fingers and toes gaining tiny black claws.
The thoughts of being human or an elf was wiped away, along with thought of modesty or good hygiene. The two goblins giggled to themselves. The truth of their existence overwhelming everything else.
"... Need more goblins," the new female goblin giggled.
"Make many more goblins," the male said. The couple crept closer and closer... and took the glowing green gem and left the cave behind.