Elves are things of myth, even amongst all the other races and cultures that exist within this world. They are the boogeymen of the woods, even to the subterranean races of the mountains and the canyons. Many see them as tales to warn others not to delve too deeply into ancient arbors. Others see them as nothing more than children's stories.
The reality is that they transcended - not above like some of the astral researchers are hoping to do on their own - but into the very nature that they worshipped. They became nearly immortal by morphing their biological selves with the biome of the forest. It changed how their minds worked, their bodies aged, and how they were able to communicate and affect change through existing plant systems, as well as a group of their own worshippers and agents. Their congregation become not just the humanoid and animal individuals who lived in their realm, but also the microbial organisms that grew on and inside of them.