Yi Lai Ke Si, also known as Necromancer in different world, suddenly entered Huo Yuhao's spiritual sea in soul fragment when he was absorbing his first Spirit Ring. When Huo Yuhao absorbed the Shen Ling Zhi Jin (Gold of Life) and created his 2nd spiritual sea which manifested as Huo Yuhao's 3rd eye, he has moved into it and remolded his soul. Although Necromancer has an immense Spiritual Power, he has no memories beyond those of his death.
Necromancer could take control of Huo Yuhao's body which radiate death aura, and can manifest a grey-gold soul ring which contains Necromancer's abilities.
In his previous world, once he was Child of Light which is character has Absolute Light element. But as his lover Fu Luo betray him, he makes a commitment to learning more about Necromancy and destroy the human world. He had achieved the highest ranking in his world (he has a divine consciousness, but his power isn't of divine rank yet) or can say he had became Quasi-god. In the planet he was living, there was no true god (he confirmed it to Yuhao). Necromancer has a ability called "Spell", a demi-plane self making which contained undead creatures, a tower which classified as divine artifact.
The deceased Holy Necromancer is from the author's Shen Yin Wang Zuo light novel. In Douluo Dalu 2 his words in chapter 36 are the same as the words he says he remembers from his last life, which can be found in chapter 659 of Shen Yin Wang Zuo.
"My hand grasps the sun and moon and plucks the stars from the sky; this world has no one else like me."