I suspect I will not have time to make a tutorial, so hopefully this will suffice. I've been trying different ways of making motion trails for a long long time for my fight animations and this seems to have been the best solution. Several empties with the "slow parent" option, with the hook modifier, with a gradient texture. Yes, it happens to resemble a certain licensed property but the sword is not important, the effect is what I spent all my time on. With the node setup you can create a flame sword, or a rainbow sword, etc. Depends what textures you use. I put this in the public domain but if you do make something with this, some thanks would be appreciated. And send me a link as well, I love seeing other Blender animations.