User blog comment:Simbey/Real Talk: Xerath rework/@comment-5514182-20140122060651/@comment-5514182-20140122231128

Perhaps you didn't know this, but PBE Xerath once had an iteration where his passive gave him Magic Pen scaling off of his Maximum Mana.

It was just too strong with his new kit.

http://www.surrenderat20.net/2013/10/xerath-magus-ascendant-rework-on-pbe.html