Yi Lai Ke Si, also known as Necromancer, is a Soul from another world that entered Huo Yuhao's body when he was absorbing his first Spirit Ring. Since Huo Yuhao absorbed the Shen Ling Zhi Jin, Necromancer has moved into Huo Yuhao's 3rd eye. Although Necromancer has an immense Spirit Power, he has no memories beyond those of his death.
Necromancer could take control of Huo Yuhao when he gave in to his rage and anger. Huo Yuhao's body radiates a terrifying aura when Necromancer took over--like the god of death.
Necromancer's manifestation is that of a grey Spirit Ring with golden patterns, and he refers to his abilities as spells. Necromancer can be considered Huo Yuhao's 3rd Spirit, but it possesses only one Spirit Ring.
It is suspected that Necromancer is, in fact, the deceased Holy Necromancer from the author's Shen Yin Wang Zuo light novel. It would make sense, as Necromancer has a Light-type power, is a master of necromancy, calls his abilities 'spells', and has died and come from another world. If it is true, then his true name would be Elux, the Holy Necromancer.