Those lines were not really invisible; but they were very thin, and as tiny as the mosquitos in the Fiend Prison. If Jing Jiu hadn't used his sword vision, he wouldn't have been able to discover them.
These lines were not employed to mount an attack, which meant they didn't have any lethal intent, but waited for the victims to rush into them.
The situation was the same as when he killed those monsters of the Snowy Kingdom in the snowland many years ago.
Yet, why were these lines so thin and sharp that he would feel a slight pain on his skin?
And how were the ends of the lines fastened if they were so sharp?
Jing Jiu, as a practitioner of the sword work, was fully aware that the thinner an object was, the sharper. On the other hand, the thinner objects were also fragile unless they had a powerful binding force inside.