User blog comment:MacTavish Shadow/Zeraf The Spell stealer/@comment-4434378-20130227164412

I'll help a bit here:

Passive:  Zeraf gains 10 ability power per stack, receiving 1 kill or assist. This effect can stack 25 times; if Zeraf die he will lose 2 of his stacks.

becomes

Passive: Whenever Zeraf has a kill or assist, he gains a stack of Magika giving him 10 ability power per stack. This effect can stack up to 25 times, upon death he loses 2 stacks.

Of course this passive needs some changing, I'd recommend changing the AP per stack based on level so like: 2/3/4/5/6/7/8/9/10 at levels 1/3/5/7/9/11/13/15/17 and capping it off at 15 stacks.