@🔥💢AndyDatRaginPurro💢🔥
Firstly, I must mention this was made in Garry's Mod. I've never touched Source Filmaker in my life (because my crappy laptop couldn't even DREAM of handling it, let alone actually run it) If you're asking about tips for how to achieve this in SFM, I have no clue, sorry.

Anyway, it's been a while since I made this, if I recall correctly, I used the Post Process Bloom effect and altered it to affect green light.
Then, just play with the colour multipier and passes (maybe tweak the vertical and horizontal blur too? Like I said, it's ben a while since I made this so I can't remember how I made it all too well)

The actual light casting on Undyne's face, neck and chest is done with a green lamp outside of the shot. This is because the soul doesn't actually give off any light, so I needed to add an external light source to give the illusion of a glow that affects it's surroundings.

Hope this answered your question and helps you out in the future!

How did you make that Soul Glow?

i think this new sfm model made undye a lot more popular, good art btw