Keystone for mana hungry mages idea

So there are mages out there that absolutely eat mana

A good example of this is anivia and Cass

So here is a keystone idea that will be decent on any mana using Mage

Arcanum core

After casting 5 spells your next AA to an enemy champion restores 10% of your missing mana and deals magic damage equal to 1%+1/as100ap of your targets total hp

This gives mana fixes to mana starved champions while adding a decent way for attacking tanks that is still countered by MR as it is magic damage still

It also forces the Mage to AA an enemy champion to get this effect, adding a risk reward element