Hiravias has a base intelligence of 55!

[Aerothorn]

I just checked his intel to decide whether to buy a helmet for him...and it's 55 base. Which explains why his lightning storm is covering the entire screen....

 

So that's definitely a bug. Also, if I use the console to change it back to normal, will that prevent me from gaining achievements?

[sparklecat]

Yes, using the console for that will probably stop you getting achievements.

 

Do you have the Drinking Horn of Moderation equipped on him?  That's the easiest way to trigger this happening, but any Intellect boosting items can do it as well, over time.  Devs are aware of the problem.