It doesn't happen all the time, most of the time that lighnting effect does nothing from what I can observe.
I'll try to get it on video but it definitely does happen. I don't know what is the chance of occurring but when it did happen it insta ripped my guy with 6.5k hp on std.
Edit: doing the video now, wasn't an instant degen. May be more of a bug, if it is not a bug, then it is pretty bullshit. Basically, what happens is normally the lightning cloud either deals lightning damage to you once, or it does nothing, or sometimes it gives you an extremely brief moment of shock status, or a combination of those.
And then on rare occasions, that same cloud will act as if it was lightning tendrils and deal consecutive damage (hence looking as if you were under the effects of degen). This damage is an AoE, spell, and it is stackable, if multiple "lightning tendril clouds" are generated on top of each other, they will all do damage to you.
The damage of these zaps vary and is decided at the time of it being casted, so if for example a cast of this hits the high roll, then all consecutive zaps from that particular cloud will deal the same damage. So if you encounter this situation on a high damage roll and multiple clouds happen to stack on you like it did on my EHC guy, then you can die instantly with pretty much no warning asides from the boss's short casting animation which isn't any different from the regular cloud.
Although these zaps are blockable by spell block and triggers effects like Aegis. Not everyone has spell block nor Aegis. But I will show you that even with spell block how dangerous this shit can get as you will see in the video that i'll upload once I proccess all that stuff on movie maker, slow computer.
Bosses (except for atziri's double flameblast) do not have penetration, not that I know of anyways. But that is besides the point. Quetzerxi's "Vaal Lightning Tendrils" for a lack of a better name is just a stupid spell. Once cast like vaal flameblast auto channels, but it hits you 4-5 times a second PER cloud, if you are unlucky and get 3+ tendril clouds stacked on you, you will be taking at least 12-15 hits per second.
If that spell was given to a more commonly done tanky boss like T16's Minotaur with the damage scaled up to that of the level 80+ zones to hit at least like 5k lightning damage pre-mitigation, there would be a shit load of people starting to complain for it would completely fuck anyone without Aegis+armor or some form of instant leech + high damage.
GGG logic: Hey let's make a spell that gives the players a sort of Russian Roulette kind of feeling, 99% it'll be survivable or harmless and then 1% of the time it'll instakill by doing a shit ton of damage in a short period of time, and of course the casting animation wouldn't be any different from the other blank shots done so there'll be no way to tell until it is too late. And to make it fair we will buff melee by another 6% so the boss dies a bit faster.