I've run into a bug on PTS while testing Vestige Difficulty. Every so often when I am fighting random overland enemies, I get a large critical damage overtime strike from my light attacks while using a lightning staff. I'm on a High Elf Sorcerer. He is wearing Mighty Chudan, Crafty Alfiq, Wretched Vitality, and Death Dealer's Fete. I also have the weapon's expert star slotted in the Champion Points menu. So far, I have only experienced this while using light attacks on my lightning staff (which is on my front bar) and I haven't been able to recreate the effect with the frost staff on my backbar.
Here is a clip where I fight various enemies. Some of them are more difficult to kill, but most of them received a very large damage over time crit from my light attacks.
https://youtu.be/hNkf9pmdcfk
It doesn't always happen though. In this clip, I fight two of the same Trolls and only one of them gets the big crit.
https://youtu.be/dVxfUFvBPlg