Gu Zhen Ren
it is not important what the immortals gu do. After gu produce ants to feed itself while year gu can upgrade itself by parallel refining. That all you need to know for the basics. The important thing for a Immortal is to develop many immortal moves base on these immortal gu's. In fact, it was never that important what the gu's does, but instead how the immortals use it. If they are smart, they can found millions of ways to use it and have it act a the core while being supported by countless mortal gu's and formations. Sure if the gu itself has a great ability it would make things easier but the user can always do other moves with it. A immortal only need to have a immortal gu that is the same rank as him and belong to the same path. Is just a tool.
The function of year gu is like woman's heart that spits poison on activating and replenishes poison by eating womens heart. Year gu spits years which are necessarily time path dao marks on using and gets small. To replenish it again you have to refine it.Years can't be used directly so you would have to use killer moves to harness their power.
As noted elsewhere, unless I've forgotten it being mentioned before, a little discomforting that the effect of Year Gu (on its own) is so thoroughly avoided. Hopefully there will be mention of it when describing the immortal killer moves too! 'equally low success rate': does this I wonder suggest that parallel refinement isn't a reliable replenishment method, and might even destroy the Year Gu? It would be nice if one Year Gu could replenish another Year Gu directly, but I suspect that's how the normal refinement takes place--for instance using multiple mortal Year Gu to combine their years and make an immortal Year Gu. That would also make sense in terms of the lasting effects of parallel refinement on normal refinement: I wondered at first if a 10-year Year Gu refined to Rank 7 would have fewer years (at first) than a 99-Year Gu refined to Rank 7, but it makes sense if a 10-year (immortal) Year Gu would need 90 (mortal) 1-year Year Gu (at minimum) as supplementary materials, whereas a 99-year Year Gu would only need 1 (at minimum). Of course, this is all only my speculation, but it would be confusing (in terms of dao mark conservation) if you could rank up Year Gu with two different year amounts with the same quality of supplementary materials and get two (also mortal, unless at different times) Gu with the exact same year amount. It has been fun to learn more about the River of Time and the concrete influences it has on the visible world! I wonder a bit if with this inheritance Fang Yuan is about to successfully unseal (and cleanse of heaven's will?) the Spring Autumn Cicada... I look forward to if/when Fang Yuan establishes a timeless base of some sort in the River of TIme itself. (I am very eager regarding this possibility!)
Wait a minute,so immortal gu can only be refined by immortal essence? Is that a thing? I don't remember any of this when FY was still a mortal and was begging for immortal essence but already have immortal gu or my memory is failing me but i really don't remember any of this in all the others immortal gu,can someone confirm it for me?