Throwing aside expendable Gu, a Gu Master's first choice of a Gu worm would become their vital Gu.
The relation between vital Gu and a Gu Master were very close; one could even say their lives were connected.
No matter which side was injured, the other side would also be implicated.
Vital Gu had one advantage which other Gu did not have — that was no matter how severe the backlash was after Gu refinement failure, the vital Gu would still preserved.
Thus, most Gu Masters would regard the vital Gu as their core Gu. Generally, the Gu Master's strongest Gu worm is their vital Gu.
Once a vital Gu forms, it would be difficult to change it.
But that was not absolute.
If the Gu Master found an extremely good Gu worm and wanted to cultivate it as their vital Gu, they could destroy their original vital Gu.
But this type of action was extremely dangerous.
