Vital Gu

A well picked vital Gu makes later development smoother. A poor one drags cultivation and can decide life and death in battle. Fang Yuan already had Spring Autumn Cicada as his vital Gu from the previous life, still rank six while he was rank one. A Gu Master and a Gu should match in rank; a rank six Gu in a rank one aperture would burst the aperture if the cicada were at peak strength. It could stay only because it was extremely weak after rebirth. ^{ch. 19}

Gu Masters hunt new recipes to raise the level of the vital Gu. No matter the fusion result, success or failure, the vital Gu never dies; at most it is injured, so long as the Gu Master is alive. Other Gu in the same fusion can still die. People guess this is because the first Gu is linked to the master’s life. ^{ch. 106} How fusion itself is done is covered on refinement.

Wei Yang tells Fang Yuan to convert All-out Effort Gu into his vital Gu before later refinement. A core that is not the vital Gu can be destroyed in a failed advance and take the whole set with it. Shang clan can help with that conversion. Fang Yuan already has Spring Autumn Cicada as his vital Gu and treats that as fixed, so he looks for other ways to keep the core alive during refinement. ^{ch. 286}

When Bai Ning Bing had to hand over Ice Crystal Gu on the battle stage, it was her vital Gu. She bled from her seven orifices at once. ^{ch. 314}

The second aperture gives the option for another vital Gu, so one Gu Master can have two. ^{ch. 493}

Throwing aside expendable Gu, the first Gu a Gu Master chooses becomes the vital Gu. Injury on either side implicates the other. After a failed refinement, no matter how bad the backlash, the vital Gu is still preserved, which is why most Gu Masters make it their core and why it is generally their strongest worm. Once it forms, it is difficult to change, but not impossible: destroying the original lets the master take a new one, at the cost of enormous backlash, from heavy injury to death. Fang Yuan keeps Spring Autumn Cicada in the first aperture and takes All-out Effort Gu as the second vital Gu, so he no longer needs to worry about losing that core to a failed refinement. An expendable Immortal Gu is a bad vital Gu, because using it up would trigger that same backlash. ^{ch. 494}

A crucial condition for becoming a Gu Immortal is the vital Gu. After rebirth Fang Yuan wanted to quickly become a blood path Gu Immortal, but it was different now: his vital Gu was not a blood path Gu. The second aperture would have given a second chance, but that blood path vital Gu was still buried in the inheritance. ^{ch. 546}

The second aperture he shatters for immortal ascension holds rank four All-out Effort Gu as its vital Gu. That Gu is of the strength path; putting it in first during the third step would help him become a strength path Gu Immortal. Because it is not rank five, he has to make up the gap with higher strength path attainment, or the putting-in-Gu step stays risky. ^{ch. 628} The crossing itself is covered on immortal ascension.