Since we all know the drama ending is a shipwreck, here's the novel ending for those who are interested:
LYF was fed the Pheonix Tears by Yu Du and Min Yunzhong which caused him to forget about his love, CZ. While he was having memory loss, he injured CZ to the point that she had to abandon her body and join with the demon sword. This made her become the sky demon, with greater powers. Before the final battle, LYF remembered her again and realized his mistake. But since the war had already started, and CZ was planning to unseal all the demons, he had no choice but to fight her. He equipped the soul binding thread on himself that inflicts the same damage on himself if he hurts anyone. If he killed CZ, he would also die, so it means he was planning on dying together.
In the end, CZ turned on Wang Yue and did not unleash the demons. She saved the entire world. Everyone was shocked and realized they had wronged her. But since she no longer had a body, she would die once the sword's powers ran out. She recalled the deal she made with Wang Yue before, that if she found where his eyes were, he would grant her a wish. She correctly guessed that his eye was the ring he was wearing, but her wish was to repair the underwater tunnel instead of getting herself a new body.
Wang Yue offered her to leave with him so she could live in the other dimension without having to die. At this point, LYF went completely insane due to his heart poison and became a demon. He refused to let CZ leave him, so he hugged her and they joined the sword together. CZ had already lost her will to live or die. She was indifferent about everything. Buddhist outlook. LYF activated the Mirror Heart Spell to purify the sword, thus killing himself and CZ. The resulting sword was one that was neither immortal or demon.
After several hundred years, we see them together again (like the extra episode). It turns out Wang Yue revived them. He's a good person after all, just like he always said. LYF and CZ still had supernatural powers but they were no longer classified as immortal or demon. They were "undefined" (just like every one of us), and they lived in the purified sword for a few hundred years before they cultivated new bodies. Happy ending with LYF and CZ talking in the street market about going back home to have sex for 2 hours.
-----
The thing I loved about the novel ending was the symbolism. CZ made the most sacrificial choice (to save the world) although she was a demon, while LYF made the most selfish choice (to keep CZ by his side even if it meant she would die) although he was an immortal. They complete the yin yang circle of being mirror images of each other. The ending is about them achieving an equilibrium state where both good and evil coexist in their sword.
The act of LYF using the Mirror Heart Spell to purify the evil aura AFTER he became a demon is also a symbolism of the dual nature of good and evil. The Mirror Heart Spell removes evil, but he could only activate it when his obsession was at its peak, and he used it for a selfish reason. That's the irony of it.