Very good, although lacking in a few areas
The part of this series that truly stands out, as everyone knows, is the level of animation. You are definitely placing yourself in the high echelon of Flash animators. Going from episodes one through five, it is clearly visible that you have spent a great deal of time not only perfecting the animations you were making, but also improving your skills which, by the end of the series, seemed to have developed ten-fold. For this, I highly respect you for your animation and art skills, both of which vastly surpass my own.
Another thing I liked about this series was the voice acting. Although it started off in the first episode as a little lack-luster, the effort and emotional emphasis was greatly improved by this one. The characters in this final title were very believable and added a great deal of depth to the narrative. Also, along with sound, your choice of music (which I believe you made yourself) was emotion and well composed. For this, I give you much kudos because I, too, enjoy composing scores and evoking certain feelings from your audience memebers is the most difficult thing you can do. You, however, hit the nail on the head. Although the music at some points seemed a little "thin" and lacking substance, it still added a great deal to the animation. The only suggestion I have is that you buy something like Kontakt Gold or the Garritan Personal Orchestra kits for whatever program you are using, if you can. This will add more depth and less MIDI sounding music to your animations.
I don't want to say, "now for the bad side", so I do want to emphasize that no part of this series was "bad", but rather in need of improvments. First off, I am a story-buff. I like narratives that are well constructed and well written. To me, it seems like you made episode one rather quick and with an undeveloped tale, that probably changed during the process of making the animation. Then, capitalizing on the popularity of the first, continued the story in another animation. (Forgive me if I am wrong, of course). Unfortunately, by the end of the series you were building off an already dull story. Cliché (which, no offense, is what this is) is OK, but I think had you planned out the entire series beforehand, you would have created a more interesting and engaging story. The story thoughout the series was very linear, with no shift in the flow of the narrative. What I mean is, in the first episode you learn what the protagonist must do, and through the rest of the episodes he just... does it. There are no obstacles that force him to rethink his position or his mission. <Spoiler> One part, however, that does contradict this is when he is forced to kill his mentor in the last episode <End Spoiler>. I can see lack of depth to the story resulting due to the limitations of Flash and the time you wanted to spend on the animation, so don't take this too harsh. Along with this, some of the dialogue could have been better. While it was voiced well, a few lines were a bit shallow.
As for this final episode, I do agree with many people that the ending was too abrupt and created a very anti-climactic finish, which I am sorry to say, takes a lot away from this series. Our hero got his ass kicked, and in the matter of a second, takes out Rhadiel in one swipe of the blade. We are left with many questions as to what happened and get the feeling that this epic battle built up, but then ran out of steam right before it could have been phenomenal. This could have been due to impatience, or perhaps you wrote it to happen this way. Either way, I feel there could have been much improvement on the ending to this series.
However, all in all, I found this a very fun and entertaining series to watch. You spent a great deal of time on on and I hope you will continue your work on a new series, utilizing the advice that I and all of your other fans have given you. Thanks for bringing this now-classic work to Newgrounds for everyone to enjoy! No matter what people say, you are a great artist and a master at Flash animations. Good luck with your next work. I await it.