The cultivation technique that he had was called the 'Death March', and it was a method that allowed one to refine Qi from the heaven and earth essence. However, the price one had to pay was rather grave.
The technique would cause the energy that one refined to be oozing of death energy, and one's life force would slowly get corroded until one became an undead.
This technique had never caught Xue Wei's eyes before, as it was quite reckless to harm oneself for faster advance. Although one's strength would rise, the price to pay would be incredibly high, but now that they were in the Ghost Realm, this ability was quite useful.
If ghosts could cultivate, then this would, without a doubt, be the most beneficial technique for them, as it would not only provide them with Qi, but also death energy.