Xerath is one of my all time favorite champions. Although the one thing i hate about him is his passive (15% of his AP is converted into armor). It is pretty much useless to a champion like Xerath. Taking the role of high damage artillery, having armor is pretty useless. If you were even able to build enough AP on Xerath (let's say hypothetically 1000 AP), the armor conversion would not be enough (only 150 armor from his passive at 1000 AP).
This new passive will allow Xerath to become a very strong glass cannon artillery unit.
Arcane Power- Every time Xerath cast a spell or basic attacks, he builds stacks of Arcane Power. Once he reaches a certain number, his next basic attack or spell cast deals true damage to the target.
Now giving true damage is quite dangerous as it does make a champion overpowered. this is why i would consider making the stack at which Xerath would have to build up to quite high. something like around 40 or 50. not to mention, as Xerath players spend more skill points into Xerath's other spells, it increase the cap on Arcane power. by doing this it ensures that players aren't going to be able to dominate the lane because of how much damage the champion puts out.