The idea here is that the immortals have retreated into… some unknown world, to avoid the violence and depravity of the humans and the fallen elves. Lillenna is/was half-human and mortal elf, so. Super lonely sad times for her.
Mortal elves dissolve in water, so their burials are conducted by rivers. The cemeteries do not hold bodies, but special seeds designed by the elders. Each seed supposedly grows into a unique flower representing the lost soul.