Kakashi created the Chidori as a substitute because he couldn't add his lightning affinity to the Rasengan. Raikiri(Lightning Blade) is simply a concentrated form of Chidori by Kakashi. [Correction] It has never been stated that Raikiri is any more concentrated than Chidori they are theoretically the same move - both lightning types, both have tunnel vision side effects [can be negated by the Sharingan], both are most critical when hitting fatal spots such as the heart. [Correction 2] Guy says that Kakashi nicknamed chidori raikiri after hitting a lightning bolt with it.