Foundation Establishment was a huge watershed for cultivators and a pivotal realm for all later levels of cultivation from mortal, to immortal, to celestial. Even heavenly sons from upper worlds who were born into immortal dynasties with storied histories did not dare play around with Foundation Establishment at all.
The reason why Foundation Establishment was so important lay in the specific methodology of its practice. Here, one's goal was to achieve two things, the expansion of the dantian and the lineage of spirit roots.
The expansion of the dantian was naturally to achieve the ability to store more spiritual energy for all sorts of purposes. The Foundation Establishment was the first and last realm where one could actually physically expand the dantian in terms of size.
