Background Information
One of the first installations of the Nasuverse and the spiritual precursor of both Tsukihime and Fate/Stay Night. The story is centered on Ryougi Shiki, a girl who, after having a near-death experience, can see the lines of death. With these Mystic Eyes of Death perception she can kill anything with just a simple knife. She's working under Touko Aozaki and solves various Magic-related mysteries. The originally novel series got adapted into a series of anime movies and it ended just recently (2007-2010). Unlike the anime versions of Tsukihime and Fate/Stay Night, the movies are made by Ufotable and do not suck.
Standing in the Battledome
As for the power standing of the series, it can be called both the weakest and strongest Nasuverse setting. While KnK definitely has some haxed or even Broken chars, they fare poor in speed department (with only one bullet timer and few supersonic chars). On the other hand Ryougi Shiki in her 3rd personality might be the strongest Type-MOON character to name with her supposedly being the avatar of a multiversal entity and claims of granting any wish or ending the world as trivial. She seriously lacks feats to back that up though and there are a few contradictions regarding her power (like Word of God stating she can barely protect herself from a Servant).