This is Electrical Treeing. Or a Lichtenberg Figure.
If you have an insulating material, it doesn’t conduct very well. But, you can force it to with absurd amounts of electrical fields. Then in the material, changes and defects cause huge localised electric fields, creating a plasma in the insulator and damaging it. Seriously electricity is terrifying.
Oh, you know what else is a good insulator? Skin. After lightning strikes people have developed these patterns on their skin as the electric field bursts capillaries. Making them the coolest scars in history.