Mononoke is a spin-off based on the final arc of Ayakashi: Classic Japanese Horror. And I have to say, it does extremely well as a stand-alone series.
The series consists of several arcs, 2 to 3 episodes each. During each arc the main character, known simply as "medicine seller", will find himself in a situation where a mononoke (vengeful spirit) is going to be involved and will kill people if nothing is done about it.
Each arc, although short, has an interesting story with a deep, underlying theme relating to the reason for the mononoke's existence. The main character will converse with the other characters involved and observe their behavior to uncover the truth of the matter. The deeper into the arc you get, the more twists there are until eventually the core of the matter is revealed. The mononoke's Form, Truth and Regret are deduced and the medicine seller will use his Sword of Exorcism to purify the spirit.
Overall the story of every arc was unique and great, with each character being in some way vital.
The art-style is certainly unique for anime, reminiscent of ukiyo-e woodblock prints. There is a paper-like filter over the screen and there is heavy use of bright and vibrant colors, giving the series a particular feel. It works surprisingly well for the horror it tries to evoke.
While I did not like either the opening or ending theme very much, the sounds used in the series are phenomenal in getting the atmosphere just right.
All in all I enjoyed the series quite a bit.