User blog comment:Deshiba/Item concept; Guinsoo's Rageblade remake./@comment-5333760-20130521060122/@comment-4022742-20130524095458

And maybe you don't understand what I mean. 10 and 15 penetration might be nice early. But late game 20% penetration is definitely better as it will probably be more then just that 10 or 15 flat. There's a reason builds into  and that  builds into  they both get an added effect of either percentage armor reduction or percentage current health shred.

Percentage. Now why isn't there flat on this item suggestion? It's because it would be WAY too strong. I already feel uncomfortable with the thought of people like getting this form of  coupled with a  and a. It would boil down to 52% magic pen. With flat pen in runes and additional flat pen on this item? it would scale out of control really fast.

The way that could be done, is if the percentage penetration from and  wouldn't stack with it. Or of the passive would be flat penetration instead, but that would make it a conflicting item, with an early game passive on a late game price.