Ok I like to play Leblanc and I am considering two different finished builds rite now, I wanted to look at some numbers to help decide, playing against an imaginary enemy champion with 100 Mag Res when we are both level 18.
Calculations will be made assuming full stacks on stackable items.
Also, Abyssal Scepter will work because it's range of 1000 outranges all of LB's abilities, so the offensive Aura will apply.
Also, for simplicity I will ignore runes, assuming that in LoL utopia, runes of opposing champions will negate each other.

One of the builds is:
Will of the Ancients (spell vamp for survivability: imagine distort in a minion mob with 25% spell vamp, or >2k bursts with the same. Also for a decent amount of ap)
Sorcerer's Shoes (movement speed and MaP)
Mejais (AP)
Rabadons (AP and amp up other AP)
Void Staff (Decent AP and MaP)
Rylais Crystal Scepter (Decent AP and CC)

One of my favorite combos with LB is to do q-r-w

So if I do this combo using this build,

I will do 245 + 0.6*(734.5), since I follow up with an r immediately, I get the bonus damge from sigil, doing 100 + 0.3*(734.5), then the r does (245 + 0.6*(734.5))*1.4, the q trigger's the repeat of sigil's bonus, dealing (100 + 0.3*(734.5))*1.4, using my w I do 245 + 0.6*(734.5).

This total sum is equal to 3064.22 damage.

This item set gives me a flat 20 MaP, and after the flat MaP kicks in, the void staff and my masteries will give me a 55% MaP, so instead of the original 100 Mag Res, my enemy champ will instead have (100 - 20)*.45, which is 36. This means the percent of damage they take is 100/(100 + 36), or 73.529

The damage the imaginary champion would take then is 3064.22*.73529, or 2253.103 damage.

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My other build, which at the moment I prefer, is:
Will of the Ancients
Sorcerer's Shoes
Haunting Guise (MaP)
Mejai's
Rabadon's
Abyssal Scepter (MaP & AP)

Redoing the previous combo, I will now do

245 + 0.6*(663) + 100 + 0.3*(663) + 1.4(245 + 0.6*(663) + 100 + 0.3*(663)) + 245 + 0.6*(663)

This pre-sum is equal to 2902.88 damage.

The imaginary champion will change from 100 magic resist to 40 first, and then with masteries it will change it to 34. Damage taken will change to 100/134*2902.88, and the enemy champion will take 2166.328 damage.

Notes of Deliberation:
Build 1's key advantages are this: MaP focus on percentage reduction means that it will be more useful against tank characters who have higher than average Magic Resist values. This lets her do high damage output against tank characters, and not just squishies.
In addition, the CC from Rylai's will help kill runners/survivors, especially when combined with Ethereal Chains. Using Distort with Rylai's to do true CROWD control can help in team fights, making LB who is notoriously useful for single target burst damage a bit more useful in team fights.

Build 2's key advantages are this: MaP focus on flat reduction means that most champions will have no effective MaP during engagements. This lets her do extreme damage output against squishies and anyone who's unfortunate enough to have less than 60 Mag Res. The above combo does over 2902.88 damage to such champions, considering champions like Ashe and Vayne have a base health of less than 2k at level 18, this is a kill burst, and if they escape because of health boost or Mag Res items, Ethereal chains should guarantee a kill. In short, Build 2 has almost a guaranteed kill burst on squishies. Using it to take out carries in a team fight can make a big difference, and using it to kill a squishy in like .2 seconds during a 1v1 engagement is entertaining.
During team fights, Abyssal Scepter's aura can be helpful if your team composition involves other AP damage.

So key points from these two long sections:
Build 1 is better for attacking tanks for high damage, but not necessarily a kill, and Build 2 is better for disabling squishies via close to guaranteed close to instant deaths. Build 1's team fighting focus is for multi target high damage hits that apply cc and getting tanks. Build 2's team fighting focus is for debuff from offensive aura and getting carries/mages.

Certainly they might fit different play styles better, but I would like to know which of these builds the community thinks is better. Also, if anyone has any other uncommon LB builds they recommend, please post it as a reply.

EDIT: Neither builds are listed in the actual order I buy the items in, and they represent final builds, i.e. near endgame. Sorry

Final note:
**** banshee's veil

Holy cow thats alot of info. I can't really say which build is better because it depends on the enemy's as well as your own team composition. This is cliche but there is no better way to say it. You already pointed out the pros/cons of each build, so nothing more to say to that.

Ill offer some other viable item builds
Have you looked into Deathfire Grasp? If the enemy champion has a tanky dps (who doesnt these days) than chances are, they will stack some HP. DFG might add a little more burst to your rotation.

How about Rod of Ages? The other day an AP Yi ( yes I know AP, not DPS) criticized my item build by saying "ive never seen rod of ages on lb". I find that catalyst is an awesome item, and really gives alot of laning power. And catalyst builds into RoA. It gives alot of HP, Mana, and decent AP. The important part is that the HP it gives allows you to survive better, and perhaps deter their focus on you.

Another item I've considered is Lichbane. I haven't tried it out myself, but with enough AP, one auto attack for ~ 300 damage doesn't sound too bad.

Considering the fact that LeBlanc's AP ratios are only decent, I would say Magic Peneration may be worth more than AP, but I'm not sure how to figure this out

Why will of the ancients???.. you don't need survivability when you have two blinks with longer a longer range than flash?

I believe the best build is Deathcap rush....

Although since I don't have HP Quints, I prefer the Hp Rylai's gives me, so I tend to get that first.

Ability tome
Mejai's*
Rylais
Sorc shoes/ boots of mobility
Deathcap
Abssyal sceptor.

that is my build... the last two item slots depends on the opponents.

The kill combo on Leblanc is Q R W E Q... you seem to be underestimating the power of E as part of your combo.

Leblanc doesn't farm all that well so you need to only get items you need, a lot of people argue that Rylai's gives you uneeded Stats (HP)... but I have AP quints instead of HP quints so it works for me... Also giving my spells a slow.. makes me inescapable.

I also find I don't need Mana.... Leblanc isn't a spammy champion she is wait for the right moment and hit them with everything... A Paitent Leblanc player can easily subsist on the mana masteries

Yeah it's true I guess there's no master item set to use, being versatile is important. The main things I was wondering though was, how useful would Rylai's be, taking account of it's fair amount of AP, it's passive, and it's health boost.

The other point I'm not sure about yet, is, when playing experienced players who will wisen up and buy a FoN or a banshee's later in the game, after a certain point the percentage MaP from void staff will far outperform flat MaP from things like haunting guise, while early game flat MaP like sorcerers staff will help get a lot of kills. Is it better to go straight for percentage, or to get flat at the start and then sell off and get percentage if/when the enemy starts beefing up?

Also, unrelatedly, I wonder if Riot made the Abyssal Scepter and Void Staff to be foils to each other; same AP, and two different types but equally useful (in different scenarios) types of MaP. Also their names are similar, Staff and Scepter are similar things, Abyss and Void are similar things.

Basically if you are planning on stacking AP, then go void staff. Its hard to explain it in terms of AP, but the pro AD ranged carries usually rush IE, then get Last Whisper for the armor penetration. What I'm trying to say is if you want void staff, then get 0 magic penetration items (haunting guise abyssal skull). If I remember correctly Flat Magic Armor Penetration is calculated before Percent MA Penetration (stupid I know). Use the gold you would've used on magic penetration on items that give more AP, so in a way this makes your Void Staff have a more gold value. You could take it to the next level and buy more Magic Penetration items, its completely to you.

Or even better here is some math!
Assuming standard 9/0/21 caster set up with Rune Page of Red Magic Pen this means you get 15% magic penetration and 9 magic penetration from the start. If you buy mpen boots (20) abyssal skulls (reduction of 20) and haunting guise (20) this means that you will have ~70 mpen + 15% mpen. This is a good amount of reduction for most typical carries, because of the fact that they don't go over 100 MR (usually). If you pick up only Void Staff as your only item then you have 40% + 15% and 9 flat mpen from runes. And if my math is correct then for Void Staff to equal the magic penetration of the 3 items will require the target to have ~150MR. I'm not sure how %mpen is calculated so I can't give you an exact answer.
The math isn't entirely accurate but should give you the general picture. You also have to take in account the cost of each item, and other things. 70 mpen is good, but thats 3 inventory space of items and alot of gold, whereas Void Staff is only 1 inv space. The other two inv space can be utilized for more AP, which is always a plus.


Rylai's is a good item, but for some reason it doesn't click with me. I have never bought it in a game, so I should try it out in the near future, but in theory it should be great. It gives good amount of AP, and a great survivability item (Slow + HP). And HP is never a wasted stat. 0 HP = 0 DPS. I guess the main reason why I don't pick it up is because I love my RoA and catalyst so much.

The enemy team knows that LB has huge burst damage, and anyone with a brain will know to focus her down first. Building survivability items will punish the enemy team by allowing your team more time to deeps them down.

As to when to pick up the item is up to you. I used to play HoN, and on the forums over there, someone did the math for an item called Spellshards. What it basically did was ignore X amount of magic armor on that target (did not bring the target's magic armor in the negatives (in a way HoN's ignore is like LoL's penetration)). The key point the person pointed out was that at any magic armor value, spell shards will do a constant % increase to the target. It didnt matter if the target had 6 armor and ignored 6 for a total of 0 armor, or if the target had 20 armor, with a total of 14 after Spellshards. In both cases, Spellshards caused an X% increased damage taken. I can't say for sure if the same thing happens in LoL, but HoN had diminishing returns on its armor/ magic armor if I remember correctly so something like this may not be far from the truth.

Yeah there is diminishing returns in LoL too, the damage received times 100/(100 + armor/mag res) is the amount of damage you take. The issue with mixing both flat and percentage MaP is that it currently does flat first, and then percentage, so in the case of 70 MaP and 15% MaP, it would take 100 to 30, and then 30 to 25.5 effective Mag Res. If instead you had done 55% and 9 flat, you would have 100 - 9, making it 91, and then 91*.45, or 40.95 effective Mag Res.

I know 70, 15 and 9, 55 aren't the exact numbers, but if we pretend they are, we find that the 70 flat will outperform the 55% until the targeted enemy has an mag res of at least 138.625, in which case the enemy will have an effective mag res of 58.331 using either set. The point is, if your function on the team is incapacitating squishy champions, it might be most effective to do a build similar to my original theoretical set 2.

The max q-r-w damage was 2902.88 damage. If we had say an enemy Ashe and we pretend she has +700 health from her items and runes and what not, at level 18 she has a health of 2402. 2402 is equal to 82.745 percent of 2902.88, so she can have an effective mag res of up to x, where 100/(100 + x) is equal to .82745. This means her effective mag res would be about 20.853, working backwards, we say that this is 85 percent of some other number, the other number would be 24.532

Then we add back the 60 flat magic penetration from items, so we're at 84.532 now. If you get all marks of MaP, then we add in additional 9, bring the total to 93.532, or around 94.

This means with the second build, a q-r-w alone will be enough to kill our theoretical Ashe even if she has up to 94 magic resist. Of course the 700 health is not exact; some Ashe's might not get any bonus health and go for straight damage output, or for defensive items they get armor or mag res. A banshee's veil, the most likely item for a ranged carry like Ashe to get is only 50 mag res. If our imaginary Ashe had one, her level 18 mag res would only be 80, significantly less than 94; assuming Ashe's veil is popped, she doesn't stand a chance. The point is, the q-r-w alone will be enough to kill a full health squishy the majority of the time. If they instead go for an FoN because of the higher Mag Res, their level 18 Mag Res will be 106. This means we will do slightly less than enough damage with the q-r-w to kill them, but as Narvuntien pointed out, I didn't even calculate the full combo of q-r-w-e-q which should be more than enough to take them out. While the q-r-w allows for a split second bomb of thousands of damage, the q-r-w-e-q will give the enemy champ (or champs) time to react. Luckily the both of the original builds involved 25% spell vamp so unless you get cc'd well, you should still be able to get the carry kill and then move out, even in a team engagement.

This means the second set enables the leblanc of being completely capable of taking out a key enemy carry in a matter of seconds, if that. In a team fight, being able to do something like kill a support character like Sona almost instantly means that your teammates can focus someone else. Getting rid of a non tank enemy champ that's key to their composition that quickly can free up the targeting opportunities for your teammates and could potentially decide the fight early

I did some similar calculations on Leblanc sometime ago (i didn't post them). But i did find that using 80MR is a better base for calculations (30base MR + Banshee's Veil), this is what most of your targets will have.

You pretty much already made your decision about the builds, 1 is better for tanky teams and the other is better against non-tanky teams. Build 2 should be your default without question, this is your job to kill the squishes. Only go build 1 if you run into some crazy tank team (it happens more often then i'd like).

I also think your putting to much focus on end-game build, especially when your playing Leblanc you don't want the game to get to this stage, the longer the game goes on the weaker you get honestly (due to the nature of team fights), you are MUCH more effective early-mid game in 1v1 2v2 and 3v3 skirmishes.

/My thoughts.

well ideally the game will end early, but when it drags on too long sometimes my team will start losing clutch team fights and getting pushed hard. Basically I want to prepare for a worst case scenario. Normally this never happens; last game I played with LB it ended pretty early, I only had Mejais, Haunting Guise, Sorcerers Shoes, and rabadons. I forget how many stacks but I was 11-3-8.
Also, in the post i made rite before yours, I mentioned the 80 mag res haha.