I've written a review for my other favorite, TTEOTM, and decided I should write one about this one (I watched it years ago).
If TTEOTM was a culmination of arcs and ideas that were exploited satisfyingly (albeit with some plot problems), this one seemed to start at a single point and expand outward relentlessly from there. The pacing is intense and substantive, the story is complex and masterful, and the acting is nothing short of wonderful. Very rarely do elements come together to form what could be an alternate universe or a chapter in history... Nirvana in Fire manages this, and is probably thus one of the best dramas I've personally ever watched.

Where to even start? Probably the story, the most wonderful thing about this drama. Although this meets the eye as the typical revenge plot with an ambitious strategist as the protagonist, there's so much nuance and many dynamics under the surface level that truly connect the dots together. How Lin Shu's history is intertwined with the present for many characters including Jingyan is beautiful, and the little twists and schemes involved (completely impossible but riveting) from how Mei Chang Su deals with Nihuang's suitor contest to how his aid towards Prince Yu helps Prince Jing is magnificent. Not only are the details critical and brilliant, they play a tremendous role in how the drama ultimately develops. Despite the numerous arcs, etc. that are present in the drama, they still fit into the narrative on how Lin Shu achieves his revenge goal. The writing in between, including the dialogue and script, excellently achieves this effect (not sure how the countless translated captions fare in corroborating the intent of the original script... watched this in Mandarin). Cleverness and wit are woven into every aspect of this drama and its characters' motivations (they all make sense given the time, personality, etc.). How the drama doesn't seem forced (besides its pacing at times) as well as the plot's fluidity is really impressive storytelling; it's also what separates this from similarly good Wuxia/Xianxia that doesn't reach its heights.

That being said, its complexity makes elements of the plot sometimes difficult to remember beyond the initial watching. That makes it satisfying to rewatch, and every rewatch I still find myself astounded by the numerous twists and ideas present. Given this, know that I watch a drama for the ideas and cleverness more so than the execution/logical sense (this one for the most part fulfills both, but there are some plot holes you find if you think carefully enough), and therefore I'd probably enjoy gripping stories much more than most/prioritize them over other things to make/break a drama.

I guess that's enough time spent on the story. I think the second thing to note here is the acting.

To be fair, there isn't much character development in most of the characters more than a change of motivations/desires, etc. I do think the characters fit perfectly with the setting and situation, but for those looking for those coming-of-age/character development arc dramas, this one is probably less so your thing... I'd push you to TTEOTM to see Tantai Jin if you wanted to see character development. The characters are mostly well developed already in this drama, and because of the abundance of motivations and schematics, it's hard to follow individual characters throughout (even the protagonist).

Given this, I don't think this drama would be a rave review for any individual actor. Instead, the entire cast does a great job portraying their parts to really make this story work. From Xia Jiang to Xie Yu (antagonists along the way), the antagonists of this drama are played amazingly. Their micro expressions, desperation, and different emotions are played realistically and believably for the plights they find themselves in. I do think a special shoutout has to go to Victor Huang who plays Prince Yu; his scenes are done beautifully to portray a pompous and crafty prince who eventually devolves into resignation. Additionally, the characters really do interact with the story and plot's purposes, something that I find often missing from a drama. The protagonists are played similarly well, with Mei Chang Su (Hu Ge) displaying the depth needed for a sick strategist who needs to finish one final task. He did remind me of Tantai Jin (Til the End of the Moon), but this one is significantly less dramatic of a portrayal (I think it's fair for the role Mei Chang Su plays) and therefore also less memorable. It's great acting nonetheless, and other protagonists like Consort Jing do really come alive with the acting of a splendid (and very expensive) cast.

Finally, I'll talk about the OST a little. I personally quite enjoyed the song 红颜旧 sung by Liu Tao (plays Mu Nihuang) and think the majority of the songs are quite pleasant. I did enjoy their melodies although I oftentimes found the singing subpar (that's okay, I do think it's more representative of the cast when sung by the cast). It's one of the great OSTs I've heard, despite having few songs. I wouldn't watch it for the OST (like I would Love Between Fairy and Devil), but it's kind of a bonus :)

Definitely the best drama I've ever watched and will be coming back to it over and over again.