Glistening in the light of Acheron, the crystalline structure of the Atrium is a fragment of Earth, carried across the void; a snapshot of green and blue amid the pale tans and reds of Tarterus, a promise and a dream.
(Apologies to GRRM and HBO)
A group of northerners want to bring the Old Gods back to the south. They grow cuttings from the white trees seeking the blessings of the Old Gods on the project, and when the saplings are big enough, they carve faces in them and secretly plant them in forests all over the south, to extend the 'reach' of the Old Gods.
The plan works of course, but the trees are *baby gods*, and behave as such when invoked.