EDIT 8/12/20: Okay, I am rewriting my entire review for this book because priest updated a whole new version of this during May 2020 which I binged in the past few days, and this version is SO much better.

Plot:
This novel starts with a prologue about Sheng Lingyuan's decision to jump into Chi Yuan (Fiery Lake?) 3000 years ago to die. In the modern time, Xuan Ji (protagonist #1) starts his new job at a secret governmental organization that deals with fantastical/magical issues. Right as he starts his job, there's a crisis - someone used blood sacrifice to awaken the demon who was sealed away in Chi Yuan. The awakened demon is Sheng Lingyuan, the other main character. Xuan Ji and he try to test each other, each with their own agenda. For the rest of the book, sinister, blood magic/blood sacrifice stuff happens as it's slowly revealed why Lingyuan was awakened, what actually happened 3000 years ago, and the past of both of the main characters.

Sheng Lingyuan is the infamous Emperor 3000 years ago who brought hard-won peace to the human race, but he was ruthless and crazed by all modern accounts (all this is made up). 3000 years ago, the world wasn't just full of humans, but other races too, like demons, etc. Many of the races were at war with each other and the human race was losing quite badly and it looked like they would be wiped out. Sheng Lingyuan was then born. When he ascended the throne, he became the Emperor who killed the demon king, stopped the war, brought peace and prosperity. He then sealed Chi Yuan, which is the source of all magic, causing the other races to die out, and the never-ending wars and power struggles between the different races to stop.

New additions in this revised version:
I personally feel that this revised version is SO much better.

- a lot of loose plot threads tied up
- many scenes were shifted around, some appearing earlier, some having been axed, some pushed back
- Lingyuan and Xuan Ji's relationship was heavily edited
- side characters had more scenes, more fleshed out stories
- more fleshed out flashbacks to 3000 years ago, new plot scenes inserted (e.g. the mermaid palace that existed in between time - super cool)

My thoughts:
- this revised version has way less 'tell' and much more 'show'. Previously, I found all the scenes set in the past really boring. It was lots of telling - this, this and this happened to Lingyuan... this and that happened to Xuan Ji... Don't get me wrong, it was all heartbreaking stuff, but the two characters' reactions aren't really captured, and priest hadn't done enough set-up to make me invested/care about the flashbacks

- relationship development, pace, dynamic and characterization. In the original, Lingyuan was woobified. Xuan Ji was awesome as a person, but he almost seemed like a side character because Lingyuan far outclassed him in terms of power, intelligence, ability to make things happen. A lot of their previous scenes were also really melodramatic. A lot of things about the two characters that didn't really make sense in the original have now been adjusted to flow better.

E.g., in the original, the narration would often say that Lingyuan was a monster, ruthless and cold, but I didn't really think so? He was ruthless at times, but it was always kinda understandable, I never really felt uncomfortable. Meanwhile, in this revised version, he did a lot of things that showed how ruthless and heartless he can be. So that epithet about Lingyuan being crazed and extreme made much more sense.

I actually really love how Lingyuan is in effect, an anti-hero, and priest wrote such a compelling and well-written story of how he went from a kind, soft, idealistic young prince into a ruthless, merciless and almost crazed Emperor. As usual, I love her insight into her characters' psyche; she portrays them in such careful detail.

- theme commenting on the imperial China's Fengjian way of life vs. modern Chinese society's much more egalitarian way of life. I complained in my old review that I felt like the original didn't really have a tight 'theme' or message; the book almost seemed aimless, which I felt was uncharacteristic of priest, but this new book seemed much tighter, like priest knew what she wanted to talk about. Also, I feel like she might have a smaller message about time, but it didn't go anywhere

What the 2.0 version lost:
- I saw on Weibo that most ppl, like me, seemed to like this better because the plot was much more complex, sprawling, and had more consistent logic. 2.0 is a dark, time-travelling, humorous and fantastical mystery/political intrigue type of novel.

- The relationship suffered a bit though. It's way angstier, like OMG, I felt like dying the entire time while reading about Lingyuan and Xuan Ji. They also had a lot of misunderstandings and way less scenes devoted to their relationship, I feel. The word-count used up for the revised plot seemed to have been straight-up taken from the ship. Really a pity because this is one of her most compelling pairings.

Priest said in her author's note at the end that she still disliked parts of this revised edition and she wanted to overhaul the entire book, but decided to strike a balance between keeping parts of the original, and revising it. There are still parts that I feel are abrupt, or don't make total sense, but oh well. The previous version left me feeling kinda disappointed after reading it, but this version is really much more satisfying. Now I feel like rereading Mo Du...

TLDR: read the May 2020 version if you want the book to make more sense, want a more complex and coherent plot. Read the 2019 version if you want more ship scenes