Tiredness? Pain? What do... these things amount to... I am only the way I am today because of him! In order to obtain the power to kill him and take my revenge, I have endured pain that is beyond all of your imaginations!! Even now, I am still plagued by this enormous, unending pain! And with every breath, this pain reminds me to never ever forget this grudge!”— Fen Juechen to Xiao Lingxi about Yun Che 
After Fen Juechen escaped the calamity Yun Che brought he ran into a coffin which had someones soul sealed inside of it. It tried to posses his body but ended up submitting to him and letting him assimilate with him and inherit his Profound Strength.
Later after Jasmine read his memories and found out that he is someone from over 1000 years ago that reincarnated, and the person he inherited his Profound Strength from was in fact his father from 1000 years ago.
Fen Juechen is being schemed against by the Sword Master of the Mighty Heavenly Sword Region and the Devil Spirit sealed within the Heavenly Sin Divine Sword so that the Devil Spirit can extract the Devil Blood inside of Fen Juechens body as to free himself from the sword in exchange he will give Xuanyuan Wentian world dominance.
He dressed in black from head to toe being both shorter and thinner than Yun Che. Long black hair that reached his waist. Stiff and pale complexion with lifeless eyes without a hint of light. Even his aura had no life in it life he was a dead corpse.
Before the Burning Heaven Clan was exterminated Fen Juechens personality was arrogant and conceited but still didn't do things that were beneath him like using Yun Che's family member as a tool to kill him or make him commit suicide.
After the Burning Heaven Clans extermination Fen Juechen became obsessed with getting revenge on Yun Che. He became very cold blooded; killing anyone that gets in his way. The only exception is Xiao Lingxi. He will listen to her no matter what and protect her with his life. She was even capable of talking Fen Juechen out of killing Yun Che. 
Fen Juechen is third young master of the Burning Heaven Clan.
After Xuanyuan Wentian's death, since his body was possessed by him, the devil's power of Yun Che was still present on him, it was so damaged that he couldn't be saved anymore.