The Tree

Sigrid: "If I stepped in. If I went in. Would it be a different place, like The Dreaming?"
Father: "The Dreaming is the underside of the leaf of your world. The veins and passageways are shared. As are the edges."
Sigrid: "And what about the ocean?"
Father: "The ocean is the stem that feeds the leaf. Brings forth all life."
Sigrid: "And the last point at which the leaf shrivels, after everything is gone."
Father: "Of course, for the stem is connected to The Greater Tree."

The first mention of The Tree as a spiritual destination came from a conversation between Sigrid Arnulf and Aboleth/Father. This was somewhat verified by Moonlight Through Rain's memories of crackpot theories and the Journal of Father's previous conduit. 
Further evidence of this Tree was found in the Map of Davokar that the party found in Aglarond, The Glittering Chambers. 
Of the location Ael Nurnan/Stone City of Bardh, the map notes:

"City and lake named the same, this is the last place the dead will see before they go to their ships and sail across the tree to find the stars. Those living here are dour and treat their roles as seriously as possible, bureaucracy for the dead never ends."

Notably, this is the first ever mention of Stars. 
All this to say, maybe the Tree is the source of life/soul stuff, and ancient elves used to travel through it to reach a special elf starry place??