So................... why is it called Pillars of Eternity?

I'm sure it said but I missed it somewhere. Is the adra the pillars? And, yeah, about the adra, was that explained somewhere? Adra stores souls? Or only sometimes stores souls when tampered with by Engwithans? Or is just a neat green rock that grows for no known reason?

From what I gathered the Adra pillars were grown somehow, either naturally or by the Engwithans. It's said to "die" and tarnish once dug out. It somehow is able to contain or transfer souls. The title most likely refers to the massive adra pillars you keep stumbling upon in those Engwithan ruins (Iovara being somewhat eternally trapped in an adra pillar, for instance).

If they're naturally occuring they might be a function of the Wheel, but I don't think so. You can ask what those pillars are at the beginning of the game and Odema goes "What, they don't got adra where you're from?" which, I guess, means it was grown to transfer or hold souls in one fashion or the other.