Question regarding chain lightning

So is this just not available at all?

I’m on chapter 5, I’ve been a mage the whole time. I’m a water mage. I’ve not been able to find this rune anywhere and it’s the only one I want to use. It’s a circle level 4 spell and I’m rocking level 5 ones. Where is it? Massive disappointment spending the whole game not getting to use it.