Bloodletting
The secret of survival for Tristan was definitely the unique trump card that he believed he alone had in this world, the Blood Monarch skill.
Besides the overpowered skills that allowed him to have a much easier time in this unknown yet extraordinary world, it also gave Tristan outstanding strength and an almost undying body. However, such unreasonable advantages naturally had its own disadvantages. And in Tristan's case was its absolute need for Blood Essence.
Every skill and spell needed Blood Essence, and an enormous amount at that.
Others could cast a spell using something called spirit, which was a type of energy that seemed to be constantly sustained by their own body; but for Tristan, he had to spend lots of Blood Essence to do the same.