The System Asked Me to Slay Demons, but I Lived Forever Because of It

Spend life energy to infuse martial arts, and you can perform unlimited deductions. Li Changfeng can obtain the opponent's remaining lifespan by killing designated monsters. The long sword was revealed in the chaotic world of demons, causing the liver and gallbladder of these creatures who had lived for thousands of years to burst! From [Qingsong Sword Technique] to [Seven Heaven-Jieting Swords], from [Eternal Spring Kung Fu] to [Ten Thousand Tribulations Golden Body]! Li Changfeng would occasionally ponder, how come this life span increases with time? (Also known as killing demons with one sword, half-stepping into immortality) (I only have one year of life left at the beginning, and the system asked me to slay demons)