Procedural Cursed Energy
Introduction
I've always wanted to make a cursed energy effect in After Effects, but it was always quite intimidating because of it's hand drawn nature in JJK. Though this method definitely isn't perfect, I think it's quite good if it's not the visual focus of the video.
Inspiration for method
This timelapse by Seter MD provided the basis for how I got the shape for the cursed energy flame. By using a number of wave warps on a shape layer, you can get a pretty decent animate flame shape.
Flame Shape
Going from left to right, you start with a vague shape, and then add one wave warp on top of it (You can pin the wave warp to a certain side if you want to, I pinned it to the bottom for these renders). You can see Seter MD's video for how they did it, but I got pretty good results with applying each wave warp to it's own adjustment layer and just playing around with the wave width/heigth, and angle.
Stroke Effect
The next step is to add a stroke effect to the flame (which is where my innovation comes in). Seter MD exports the animated shape as a PNG Sequence, and uses an art program (photoshop in this case) to draw the stroke effect. This is an especially good way to do it, because art programs have tons of brush presets that allow you to customize exactly how you want it to look, and also mimics the way that they probably animated it in JJK. However, I hate myself and love procedural effects, so I made a procedural work flow to get something similar but at less quality but for significantly less work if you need a long clip of it.
Main Stroke Effect
I made the stroke effect by duplicating the flame shape layer, and setting the stroke to be solid black and thick. Setting the fill to transparent then gives you an animated stroke that you can edit independently and then overlay on top of the flame shape.
In my case, I used roughen edges on the stroke and animated the evolution parameter to achieve this look. I also applied a glow effect to provide a background for the stroke to contrast against, and a gaussian blur to obscure the edges of the flame shape under the stroke
Secondary Stroke Effect
For the secondary stroke effect, to get whispier black edges, I duplicated the previous stroke effect. Before the wave warps, I added a black solid with a fractal noise effect and set the stroke layer to to have the fractal noise solid as it's track matte. Then precomposing all of this and then adding another roughen edges effect to the secondary stroke gets you the final look. I also changed the wave warps so that the secondary stroke doesn't have the same movement as the rest of the flame.
Final Thoughts
I'm not sure if this is a good method, but it's a pretty convincing result, especially if placed in the background. You can also change the pin of the wavewarp to change it to be a free floating flame, more similar to what you would see in JJK. Since I designed this as a background effect, you can also change the wave warps to be more dynamic and have the flame move around more.