As a mage walks through the forest a sprite appears on a branch in front him (or her). The mage will be the only one that will be able to see the sprite. The sprite demands that the mage release him from the forest.
(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.