Hey guys
(Ok, so this is my first tutorial so excuse my bad english and my style! I will definitely upload more in the future!)
Today i want to show you how to do a sword slash trail in Blender Cycles.
This is verrry easy
I used a Minecraft Rig, because it is easy to handle, animate and editable.
Ok. So i opened up the Minecraft Rig.
Then i created a simple animation. I wont explain that, because you probably already know how to do that.
I dont know how big this image is, but in the viewport it looks HUGE! Sorry for that.
After that switch to Compositing on the header.
Now you klick on “Use Nodes” in the “Compositing Nodes” Tab.
Now we have to tell Blender what Object should create the trail. For that we take a look on the right, we klick on the “Render Layer” tab and we open the “Passes” dropdown menu.
Can you see the “Material Index” button and the Vector Button? Klick it.
Next you select the sword. You go to the “Material” tab, in my case there are two materials, and you open the “settings” dropdown menu. On the bottom right you can see a number calles Pass Index. Switch that to 1.
Now lets go to the compositing window again. At first we have to render a frame with a lot of motion in it.
Then we do the following setup:
What this does is very simple. At first the “ID MASK” grabs you sword in form of a black/white image, so that the computer knows where it should add the trail.
Then we add a “Vector Blur”, which adds motion blur, only editing the BW Image of the sword. We use this to create our sword trail effect.
After that it adds the blurred, white image on top of the original render. But in this case our sword would be white if it stands still. So we use the “MIX Node” on top to get another copy of our sword. Now we just have to add this copy over the blurred effect.
You can change the color of the trail by changing the white spot in the lower “Mix” Node.
You can also increase or decrease the effect by playing around with the “Add” Nodes.
The final result looks like this:
If you can tell me HOW THE HELL I CAN MAKE THESE PICTURES SMALLER then tell me 
Anyway, i hope i helped you out a bit and i will see you again in my next tutorial 
