This is the final effect created with the subsurface scattering effect I posted about earlier.
I started by creating an animation clip that changed the ice amount parameter of the shader over time.
I then added a quick particle effect that consisted of a glow, shiny dots and swirls. (If anyone thinks of a more professional way to talk about VFX do let me know! “Shiny Dots” is a phrase I use far too often at work!)
The glow used fairly high emission and low lifetime, plus size over life, to give the impression of light flickering. It just uses the default particle material, which works surprisingly well for effects like this.
The dots use radial velocity to pull them in, with orbit added on a single axis to give additional unnatural motion.
The trails used the texture below. Initially designed to be chunks that flew outwards, more similar to my concept image above, it worked nicely as a trial instead so I kept it. The gaps give me the nice tearing sort of effect in the swirl, and the details look far less blocky and messy when stretched out. This particle system also uses orbital and radial velocity.