[ The Origins ]
The tree is thought of as a sort of Diety, or that there is a god that oversees it. And it could just be a myth but the different parts of the tree made the people, or set the seeds to bring them into existence.
The merpeople grew from the roots, the tree's magical energy mixed in with water, and as the water seeped through the ground and out to the ocean, people grew. Slowly, but surely and made their home in the water from which they came.
As the tree shed its bark it mixed in with the rocks of the Earth and melded together, and over time people were carved out of the stone. They did not just live amongst the stones, they made their home on the land, living under the god's tree and branching out to live elsewhere, in the mountains and by the shore.
And as the wind blew through the leaves something special happened with it, a spore, a spark of life mixed in with the feathers of the birds that already lived on the planet. In a burst of downy feathers the first harpy came into existence. Wild and free, they could walk and perch amongst the trees, but instead decided to take flight like the birds around them and took to the sky.
[ The Treaty ]
Long ago, in order to return peace to the land, the leaders of each race gathered together and created a peace treaty. They split the crystal, within the sacred tree, into three parts and went their separate ways.
The harpies took to the sky, using the crystal's power to break their land away and suspend it in the heavens. The merpeople left their lake homes and used the crystal's power to purify the oceans and make themselves new homes. The trolls remained behind, leaving the last of the crystals hidden inside of the tree. They protected the tree and, in return, the tree's crystal provided them with fertile soil, so that their gardens would flourish.
Peace remained a constant thing in the lands for quite some time. Interactions between the races was (and still is) limited. With a few, rare, individuals interacting outside of official business