i noticed all the armor and magic items having their pen listed under unique passives. 
except for maw of malmortius. so, im wondering if the armor pen stat on 
does actually stack. thanks in advance for the help :3
Critmaster Garen4 votesthey probably didnt care because youre dumping a lot of gold into the other passives- stacking them makes it inefficient.
Meep Man3 votesI think they said since it was underlined, it couldnt stack with ITSELF. I can stack with Youmuu's I think for 30 armor penetration. I've been doing some funny "Armor Buster" kits on high physical damage dealers. Items like 
and the Precision mastery give a lot of armor.
Such as 
or 
Renascent3 votesChecked it out in a game: Maw of Malmortius Armor Pen can stack with more Maws.
Neo Cyrus2 votesIf it's not unique then it should stack. Then again some things even labeled as unique could sort of stack, like old old WotA. Some of its AP was an aura, and two of them could overlap.
Reaper Soraka2 votesI think armor pen and magic pen can stack, but armor pen% and magic pen% do not stack.
Maw + Youmoo's + Mortal reminder(or giant slayer) + cleaver should all stack.
Youmoo+Youmoo would not. Mortal+Mortal would not. Mortal+Giant slayer would not.
Jungle Lux God1 votesIt's not a named unique passive, so it will stack, but not with another Maw of Malmortius.
TheProAndrew1 votesyou lie, u can stack because it is +15, not unique passive
