User blog comment:Hamengeri/Fin, the Silent Assailant/@comment-5663427-20130926093141/@comment-10256611-20130926100135

Not every champion uses AP as well, and deals 80% of target's AP in magic damage. :)

At first it was bonus damage, but it was very low (in most cases), so I merged it with old Energy Stealer and made it a toggled skill. Btw, you can toggle it when fighting manaless champion ;)