The instant I saw the power sword painted up on the back of the Space Hulk booklet, I knew I had to try and duplicate the effect. I think I'm fairly close here, close enough for tabletop results anyway.
Here's the step by step process I use to get the results. Keep in mind, this is the basic version, you can change colors and such for different end results.
One word of warning, you will need a fine detail brush and a steady hand for this one. Missing one of those and you might be better off not trying this one.
Here are the paints I used. You can use anything you want, the idea is that you match your wash to your highlight color and your highlight color is a fairly bright color to start with.
1. Basecoat the sword black and highlight the edges of the blade with your highlight color. Do both edges and the blade surface down the middle of the face of the blade.
Start at the tip of the blade and feather the highlight down.
2. Add the"lightning." Starting at the top and working down, Make a series of short, ANGLED strokes. No curves here. You can connect some of the angles to the edge of the blade and some to the middle line you made in step one. As you progress down the blade towards the hilt, you'll want less and less lightning.
3. Wash the sword blade. What you're trying to do here is darken the highlight color slightly... not too much, but enough that when you get to step 4, you'll notice a difference.
4. Add the power effect. Here's where go back and add some of the original highlight color back into the lightning. I redo the edges of my blade and most of the intersections along with the thicker lines at the top.
It's really a lava type effect, in the areas you have wide lightning, you're going back in to show that the center of it is a little bit brighter and "glowing."
And that's it.
Like I said, you can vary the colors you use to create different effects. For example, on my Deathwing, I used a dark grey sword with a light blue highlight and a Devlan Mud wash.
I do have an additional method that is similar.
The idea here is to pick the color of your "lightning" and then pair that up with a wash. In this second example, I made the lightning purple to tie in with the theme of the army. I really used the purple on this guy as a spot color instead of a main color like it is on the rest of the force.
This way, he's still tied in with the army, but is able to stand out wearing the classic Librarian blue.
I went with two purples for the lightning in this case, a warm one and a cool purple (Vivid Violet by Americana and Lavender by Folk Art respectively).
Getting some color added to the background
By taking a base color of P3 Beaten Purple and adding it to the side of the blade, I'm able to give it some life instead of just being black primer. Overall the blade will remain dark, but I was looking to add a little more emphasis to the sides.
Adding the lightning to the blade.
I started with the warm purple and added a series of squiggles that originated from under the wings. A quick wash of GW Leviathan Purple followed by repeating the process using the cool purple. Each pass I made, I brought my lightning out a little less. This made the finished lightning appear to be fading away the further it gets from it origination point.
Getting just the right look.
I keep repeating the lightning/wash method until I have the look I like. Sometimes you can get it in a couple passes, sometimes it takes a bunch. It all depends on how high of a contrast you want in the end too.
So there you have two methods for creating that "lightning" power weapon effect. With a variation here and there, you can create just about any type of lightning effect you can think of. The trick is doing some experimenting before you work on your model.