Abyssal Mask gives a lot of Magic Pen

Crispy0Snake·May 25, 2019·1 votes·1,595 views

Turns out this item gives 13 magic pen plus 13 for every 100 MR your enemy has, or has left after your actual magic pen has been applied (math below)

Magic damage is multiplied by 100/(100+MR) and that is the damage the victim takes

So you need to ask how much do you need to reduce MR by (x) at any given MR (k) to increase magic damage by 15%

100/(100+(k-x))=115/(100+k)

(100+k)/(100+(k-x))=1.15 100+k=115+1.15k-1.15x 1.15x=15+0.15k x=(15+0.15k)/1.15

x~~13+0.13k (as above)

x=3/23*(100+k) (simplified)

Edit: Ofc if the enemy isn't within the very small 325 range you get no benefit from this

2 Comments

MordridtheBlackMay 25, 20192 votes

uh...no where it the tool tips does it say that

the closest thing is the "Nearby enemy champions takes 15% more magic damage."

this does not imply pen in any fashion

Crispy0SnakeMay 25, 20191 votes

Yes, the magic damage enemies take within the 325 range is increased by 15%

My question was: How much flat pen results in the same 15% increase in damage?

Now, that "magic pen" is applied after the Void staff and after Sorc shoes/Obliv Orb/Morello, because we need the MR that's left to work out the EFFECTIVE mPen you get from Abyssal mask

So if your target has 100 MR and you have 33 | 40% mPen, they then have 27 MR left, if you also have abyssal mask, then they effectively have ~~11 MR. It works out nicely as long as k>1300/87 (if you have 33 | 40, they need 80MR or more for this to work). 80MR is reduced to almost 0

You can also change it from flat to effective % magic pen

e%mPen~~(1300+13k)/k=1300/k + 13

Remember k>15 for this to work. This means that you get 100% to 13% magic pen depending on if they have 15 to a very large amount of MR