This is a very old Blender Guru tutorial, and as a result, it was done using Blender Render.
A Cloud texture was used with a material that has emission and transparency to give it a whispy sort of look as the lightning bolt fades. The fading in and out is controlled via the texture’s brightness. However, textures can’t be used in the same way for Cycles/Eevee, and I’m not the most confident with nodes. How could something like this be replicated?