Hmmm. Based on your reply on the slime, I want to run my idea by you before I fully commit to it. 

My idea was there are a group of people like druids who had found something like the world tree in Norse mythology. It grew to embody multiple dimensions, worlds, whatever you want to call it. After having lived for several eternities there comes a time where it wishes to procreate, much like other plants would. But given the immense power, the tree blesses one of the druids the seed within their person, guiding them to seek new soil where it can sprout and grow its roots and canopy into new worlds beyond the reach of the current world tree. A person with all the knowledge and power of the world he came from, but the experience of a man whose entire existence is not even worth noting in the life times the world tree existed.