Void Staff: The Forgotten Mpen Item

Penetration Mechanics Help More than Bruisers

One of the under-the-hood changes of Season 3 is the change in resistance penetration's order of operations. In Season 2 flat penetration was applied first followed by percentage penetration, while now percentage penetration is applied before flat pen. 

Season 2:      EffectiveResist = (Resist-FlatPen)*(1-PercentPen)

Season 3:      EffectiveResist = Resist*(1-PercentPen)-FlatPen

The old penetration system made percentage penetration do less for you for each point of flat penetration you already had. While not the end of the world, the inefficiency of stacking flat and percentage penetration did make a difference, and League is won and lost by small advantages - for instance, merely owning a Brutalizer made Last Whisper give about half as much additional armor penetration to lower armored targets in Season 2. Over time, people learned to generally avoid building both Brutalizer and Last Whisper, and mixed penetration AD builds became rare. 

What's odd is that while the AD camp realized this and avoided building mixed penetration, the AP players would build mixed penetration extremely often. Not only do mages typically run flat magic penetration marks on their rune pages, but they also build Sorcerer's Shoes nearly exclusively which gives even more flat mpen. Despite this, Void Staff remained a common late game item to deal with any amount of MR stacking. 

When the reds spoke from the heavens and declared the reversal of penetration calculations for Season 3, there was much rejoicing from the top laners who were excited to build a Brutalizer for early game power without lategame penalty - but where was the commotion from the mages? In theory, having percentage mpen apply before flat mpen means that Void Staff works with the flat penetration from runes and Sorcerer's Shoes rather than against it. Maybe it's a really negligible improvement, or maybe it's a huge deal that nobody's latched onto yet. Let's find out!

Less Talk More Math

I'm imagining three scenarios, all of which give us a partial picture but together will let us draw some assumptions about how strong Void Staff is. First, we'll look at the numbers and mechanics of Season 2. Not only will flat penetration be applied first, but Void will be calculated with 40%, mpen runes will be at 9.85, and Sorcerer's Shoes will be back to 20 mpen. This will let you compare how strong Void Staff is against a given foe between now and a month ago. Perhaps your mental cutoff point for "they have a lot of MR now, I should build Void" should change between the seasons. The second scenario is to use Season 3 numbers but Season 2 mechanics. All the item/rune values will be the same as on the live server but flat pen will still be applied before percentage. This is mainly to sate my own curiosity about how much mages gain from the pen ordering changing. Finally, the third scenario is what you play with everyday - S3 numbers and S3 mechanics. 

Where to start? Let's first assume you're a mage duking it out in midlane. The vast majority of AP mids have 30 starting magic resist and do not get MR per level on their base stats - it's a tough life being ranged. Furthermore, if your opponent opts for AP glyphs and points in the utility tree, they'll have no bonus magic resists from either their runes or masteries. That 30 starting MR is all the magic resist they'll get all game until they buy a defensive item or a teammate gets an Aegis. How much effective magic penetration would a Void Staff give you here? This is the lowest magic resist target in the game, and should paint the most dire picture for Void Staff since we're taking a percentage of a very small value.

• Scenario 1: S2 Numbers and Mechanics. The higher values of the magic penetration marks and Sorcerer's Shoes really make a difference, cleaving off 29.85 out of the 30 magic resist before we even consider percentage penetration. Overall, this makes the 40% pen on Void Staff nearly entirely useless, giving only 0.06 additional penetration. If going for dueling potential, Void Staff was a terrible buy in Season 2. 
• Scenario 2: S3 Numbers and S2 Mechanics. This is a non-realistic situation that I'm just using to compare the significance of the change to the order-of-operations. With the reduced flat penetration values on runes and Sorcs, there's more left over to take a percent of and Void Staff actually reduces 2.31 of the opponent's magic resist. 
• Scenario 3: S3 Numbers and Mechanics. Calculating percentage and then flat now gives a resultant MR of... -4.89? Oops, we're doing true damage! Adjusting the final magic resist to zero (you cannot penetrate past true damage) Void Staff is contributing 9.66 points of mpen but we are wasting some of the flat pen on runes/Sorcs by going into "negative MR". 

Midlane Matchup Summary

Trying to draw conclusions from the table above is a little misleading because of the "negative MR" in the third scenario. If you redo the same calculations for 39 magic resist -- the point where you're no longer going hitting true damage and hurting your flat pen's effectiveness -- you would see that in Season 3 Void Staff cuts through 12.56 magic resist compared to a paltry 3.48 in Season 2. That's a 350% increase in Void Staff's usefulness against low MR targets!

If you plot the percentage increase in Void's effective penetration versus magic resist, you can see that the change in penetration order-of-operations gave Void a huge buff against lower MR targets. It still levels out to the same amount of penetration against tanks (since the flat pen becomes insignificant compared to 35% of 150 or 200 magic resist) but it's not unreasonable to get 150% more penetration than in Season 2 to most of the enemy champions in your game. 

The really interesting thing is how well Void Staff now matches up to Haunting Guise. At 47 MR you'll start getting more penetration out of Void than the 15 flat magic penetration on Guise. And that isn't a high cutoff! Melee champs already get scaling magic resists and often run scaling MR glyphs, putting them at 77 MR at level 18, and even a "squishy" mid would surpass that cutoff simply by running flat MR glyphs or by picking up a Chalice of Harmony. So more often than not, Void Staff is giving you more AP and more magic penetration than the "midgame powerhouse" that is Haunting Guise! Not to mention the fact that Void scales infinitely better than Guise as the game progresses and enemies purchase some resistances. 

Conclusion

For the sake of keeping this post to a digestible length, I'll wrap things up here instead of endlessly rambling. 

Primarily because of the order-of-operations change on magic penetration calculations, Void Staff has received a significant yet silent buff against low MR champions. It should no longer be a semi-niche buy only contemplated when facing 150+ MR targets. It gives higher pen than Haunting Guise / Liandry's for the vast majority of real world targets, overall outdamages any Ability Power item or item combination at its price point (no exaggeration), and keeps your damage relevant even against the tankiest of champs. Consider it.