Terra isn’t in fact a kingdom at all. Not as we could think about a kingdom. It’s just the name elves gave to the territory they had to separate from human influence, for it not to be destroyed. They call Terra the whole world, Natural World, and so they call Terra to their domains.
Elves lived peacefully with humans for a long time. They exchanged resources and even celebrated several parties together. But that was when humans respected Terra – Nature – and her domains, when they needed nothing from her but some wood and Stone to build their houses.
But with time, lords started building castles, great walls, and humans needed more and more natural resources, and tenseness between them and the elves – kings of the rest of Nature beings – grew up. First, laws where made to limit explotations, but they were broken by many people. Finally, elves decided to create a frontier which humans wouldn’t pass without being attacked. That would be named the kingdom of Terra by humans and the rest of the world.
The forests also suffered demons attacks, and started to perish massively. Even the goddess Terra herself interfered, and forest animals and beasts appeared in elven towns, not to attack them, but to help defending them. Since then, elves, werewolves, tiger warriors, rhino warriors… every forest being began fighting for their home together.
And centuries have past, and they still resist Hellscion attacks, like humans, because they stand together. Although Terra hasn’t got walls, and goddess Terra needed to help a bit more her sons, but that’s another story…