Quote Originally Posted by Assertn View Post
The stuff about Shiki was mentioned in the manga, and the actual fight was shown in a tangent manga mini-arc, although really the whole thing was pretty much thrown in by Oda to support the movie he helped write.
Not helped, actually wrote

And Shiki's character had already been planned by him for a while, Oda was just thinking on how to use it and then the movie plan came along.