Maw of Malmortius is a lie.

http://leagueoflegends.wikia.com/wiki/Maw_of_Malmortius

"Maw of Malmortius helps attack damage-based champions against enemy teams with high amounts of magic damage."

https://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2661576611/232207218?tab=overview

HeLpS aTtAcK dAmAgE-bAsEd ChAmPiOnS aGaInSt EnEmY tEaMs WiTh HiGh AmOuNtS oF mAgIc DaMaGe.

xd