Onmyō Taisenki may be considered just another monster series, but in reality this quite unknown and as of yet not-released-out-of-Japan 2004 anime is actually able to carve itself a quite unique place in the genre, in that, while exploiting mechanics already well extablished, it does so by going into the world of Eastern philosophy and mythology.
This anime series is a textbook case of what I just decided I like to call "Digimon syndrome": starts off as a mildly boring, mildly childish succession of random battles, in a "monster-of-the-week" format, while extablishing the characters and getting the stuff running, only to suddently take off around the 10th episode, starting a crescendo in quality which just doesn't stop until a wonderfully epic, nostalgic and ultimately satisfying finale. At the beginning there isn't much story going on, except the information that there is a war between two styles of Onmyōdō users (a mix of esoteric cosmology, occultism, divination et cetera, which takes here the form of people fighting demons and such through a "contract" with spirits called Shikigami) and a sensation that there's something more going on beneath the surface; as the story unveils, it becomes more and more interesting and involving, gaining a complexity the watcher wouldn't have expected at first. There's full of unexpected plot twists, and even a couple of elements worth some reflection. Note that the story in itself, while being very well told, is not particularly original: what makes it so good and original is the very same factor which makes it almost impossible to translate and adapt for Western audiences outside of fansub: the way it's so deeply permeated with references to Taoism and Onmyōdō. The cosmic dual forces Ying-Yang, the five elements, the demonic portals, the world of demons, the role of seasons and their balance, Chinese geomancy (the trigrams are an essential part of controlling the Shikigami in battle)... there are also references to Heian period Japan to spice it all up some more. Of course, a good deal of the time is spent on the fights between Shikigami, and this element too follows the same trend as everything else: starts off "meh", and grows to unbelievable highs of awesomeness which will make you go "Fuck yeah!" with heavy metal horns high up in the sky.
In once again "Digimon Syndrome" fashion, the characters start off as basic stock types and grow deeper and deeper as the series goes on, not only through development but also, and most importantly, through backstory exploration.
Riku, the protagonist, is not your typical shōnen hero, right from the beginning he's closer to a Shinji Ikari, but less original and more loveable; his relationships with other characters (expecially his "we're ONLY childhood friends! Really!" childhood friend Momo and, of course, his partner Shikigami Kogenta) are very interesting; Kogenta himself is also interesting and loveable; special depth is given also to the hot-blooded and hammy Yūma Asuka, his little brother Sōma and the mysterious and sneaky Masaomi. Among the villains, the only one worth serious mention is the main and final one, but just naming him would be a spoiler so I'm gonna have to refrain, but let me just say that you'll find yourself symphathising with him as much as with the heroes, and even think that he may be even right to a degree. And that, in my opinion, is what makes a truly great villain. The cast of secondary characters, while much less developed and more "typical", if you wish, is still likeable with well defined and involving relationships evolving between them.
The art is decently solid, while in no point astoundingly great. I give praise to the design of the various Shikigami (which is really good even if a bit too "modern" for those who should be the ancient spirits guarding the 24 seasons), to the battles, and to the summon sequences. Everything else is just OK, there are no quality drops, there's nothing particularly stylish or anything, but it does its job.
The music doesn't really add much to the scenes, for the most part it's hardly noticeable. The fact that it doesn't RUIN the mood is already positive, I guess, but some great moments, if rightly punctuated by some better music, could have been much more. The first OP (Kimi to Nara) has a catchy and melodic chorus (yeah, it IS on Pachelbel's canon chord progression, if you were wondering...) and lyrics which I find fitting to the relationship between Riku and Kogenta, while I found the second OP of lesser impact, even if by no means bad; the first ED is on equal level, but the second ED, while certainly commercial and poppish, is much more impacting and memorable (probably due to, again, Pachelbel's Canon).
The entire cast does its job well, but there are a few of them who really stick out as particularly good. I know I may be licking too much of his ass in my reviews, but... the protagonist is Jun Fukuyama; the one we know mostly as the blue-blooded hammy motherfucker Lelouch from Code Geass, here takes the role of an insecure and clumsy young boy, sporting one moment a soft, effeminate voice, the next a convincing crying and emotional voice, and the next a screaming, angry and even scary voice which manages to NOT sound too much like Lelouch's and to NOT sound hammy. Sayaka Aida's performance as Byakko no Kogenta is generally good and funny (especially noticing how she sounds manlier than Fukuyama at times), but when the emotional moments kick in she's the one who drove me closer to tears. Kenta Miyake and his explosion of pure hammy MANLINESS as Byakko no Rangetsu are unforgettable, as is Masashi Yabe as Akagane no Isoroku. I also found a lot of depth in Makiko Oomoto's performance of the villain I mentioned erlier, and in Takuma Takegawa's of Masaomi, who really manages to convey everything this character has to offer (which is A LOT, considering the fact that he plays both the tricker and the tricked, the confused and the supportive, the hero and the villain, and I'm already saying too much).
Overall, for a monster series this one really shines in originality of setting and depth of characters, and can provide a great deal of fun and involving action. The problem is, the very same elements which make it original also make it hard to fully comprehend and enjoy for Western audiences. Those who are geeky about Eastern philosophy, Japan history, Taoism and such, can probably find a nice treat in this series, but I would similarly suggest to those who aren't, who are not interested and/or know nothing about it, to avoid this one, because without a few basic knowledges of these matters you'll find this anime lamer and more childish than what it actually is. Unless you are willing to take the occasion to to some googling about the subject, of course.