cultivation through skin making

Reborn in the immortal and demon world where righteousness and evil contend, Song Yan is kidnapped into the Puppet Demon Sect and assigned as a skin-making servant.
During working, his system unexpectedly awakens, allowing him to enhance his lifespan and comprehension as long as he engages in skin making.

[You absorbed the remaining life span of a stubborn donkey: 10 years]
[Please choose the lifespan to invest]
[You invested ten years of lifespan to comprehend Profound Root, you continued to lie there, being penetrated by ghosts...]