To this day, it seems that this game is finally receiving the recognition it deserves by having a cult following mostly due to its sale on gog (unfortunately it doesn't seem to be on sale there anymore). I believe this fits the criteria of becoming a hidden gem, and deserves to eventually be considered a classic.
This game reminds me of the Souls series. Both are known for their extreme difficulty when it comes to combat. One mistake could easily mean death. However, I find this game to be even more complex than the Souls trilogy, even though it was released 8 years before.
The complexity comes from the combat system. On top of the regular hack and slash commands, you have advanced combos reminiscent of fighting games. The damage system of the game is where it shines. There is a dismemberment system similar to Dead Space. This is what makes the game incredibly intense. It emphasizes the danger involved with swordplay. One false move, and not only will your health be deteriorated, but you may even lose your hand that's holding the sword. At that point, you're simply screwed. Of course, you can dismember the enemies too in the same way. Because of this, the combat is intense, frantic, and risky. It is simply the most intense experience I've had playing a video game.
There is also a variety of playing styles. Not only do you have a decent array of weapons with different fighting styles, but you also have four characters to choose from. Each character has their own pros and cons, and can provide replay value due to its variety.
There are a few minor flaws. However, I have not incorporated them into my score, as it is simply the flaws of the time. The jumping animation can be quite awkward, but I find that you rarely need to do so. The graphics, although decent when you consider when it was made, did not age that well. However, there is a mod available through a quick google search that makes the game almost gorgeous.
In short, this cult classic is a MUST if you enjoy the Souls series.