Yea, magic pen is over looked at times, but vastly superior for champions who do not have high AP ratios on their abilities, such as fiddle sticks especially.

^throw a Soraka in that mix for more hilarity. Spam Starcall a bit and you'll get them WAY in the negatives.

I posted this in the LOL forums, I wonder, what you think about it?

It's a clarification on the role of MRes and MPen.

Magic Penetration will never drop your enemy's resist below 0. Magic Resist Reduction (such as Amumu's and Fiddle's passives, as well as Abyssal Scepter) will. I run a Karthus build that starts with Sorc. Boots and Haunting Guise, which, combined with runes, gives 49 flat Mpen.

The game calculates like this: Flat Magic Resistance Reduction, Flat Magic Penetration, Percent Magic Penetration.

With his wall and Abyssal Scepter, by the end of the game, my Karthus has 55 Magic Resistance Reduction, 49 Flat Magic Penetration, and 15% Magic Penetration. Even if the opposing carry buys magic resist, this is normally enough to rip right through it...

Using the above numbers, let's take a look at a battle between Ashe and Karthus:

Ashe, if she buys NO magic resist has, I think, 30 native Magic Resist. At level 18, my wall has 35 Magic Resist Reduction, and Abyssal Scepter gives 20 more.

Ashe, being a physical carry, is overconfident, and I snag her in my wall. At this point she has -25 Magic Resist (thanks to everything) and my spells calculate as such. ( 1-(-25/125) = 120% ) Which means that my AOE, which normally does 273 damage per second, does 328 damage per second, and my bomb, which does 510, now does 612. I can cast one bomb per second- ish, so my damage per second vs this very foolish Ashe is 940. My ult hits her for 1128 damage, which is just under half her health.

All these numbers are calculated based off of 650 AP- a conservative amount of AP for Karthus.

Now, same battle vs Garen- 120 Mres. It's dropped to 65 by my wall and the scepter, which gets rid of the reduction, and the Penetration gets rid of another 49, which drops it to 16, and my 15% penetration drops that to 13. Given this, you do 87% damage to him, meaning my AOE does 240 damage per second, and my bomb does 449, making it 690 damage per second.

Finally, we have Blitzcrank, who gets Banshee's Veil against me.

80 Magic Resist, to 25 by wall and scepter, then the other flat magic penetration eliminates the rest of it, but not dropping it below zero, causing my abilites to deal normal, unaltered damage. ( AOE is 510, 273, and ult for 940.

Now, if this Blitz is out of my Scepter range, or I have just died (so Abyssal Scepter doesn't work anymore) and he's still under the effects of my wall (which is why when you die with Karth, it's the FIRST spell you slam down) my Ult will work like this: 80 Magic Resist - 35 for wall - 49 flat means I still hit him for full damage. This is why Karthus works so dang well with Amumu and Fiddlesticks.

With Karth, Amumu, and Fiddle together, along with a single Abyssal (because only ONE works- UNIQUE passive, remember?) it's 35+20+35+10 = 100 Magic Resist Reduction on any target amumu hits with his melee. The number is 65 for everyone else. Basically, here's where it gets stupid. Those numbers for ashe basically go to the entire team and all three of us magic damage dealers wipe out four of the five person team and are dealing full damage to the remaining tank after about two seconds. Amumu pops his ult, Fiddle Ults in, and Karth just walks around with his AOE on and uses his wall. If someone manages to survive, Karth takes them down with his Ult, even if Karth happens to die in the process of the teamfight.

Sorry for the book. I hope this helps, and gives you an idea or two.

Thanks! And I didn't think of that, very good point. Selling the haunting guise for void staff is a very good idea for late game. Adding it to the guide!

I like it, and a lot of it makes a lot of sense. The only thing I would consider is the enemy team comp. -89 magic pen is a MASSIVE boost to damage, and I totally agree, magic pen over AP any day if you can handle it. But in the large scheme of things, not many people build magic pen outside of a Banshees Veil. So maybe by putting Haunting Guise early into your build so that you can get a high magic pen to magic resist ratio and some health early game, then sell it later (since it doesn't build into anything) and then build an {Abyssal Scepter} where you normally get the {Haunting Guise}. This will give you some AP, the same magic pen that you get from {Haunting Guise}, an aura that gives extra damage to the other casters on your team, all while increasing your magic resist and survivability. But the route sounds good and the explanation sounds good and your reasoning is pretty sound. +1 from me

If no one else on your team picks it up, I suppose an ancients would be viable in the scheme of team fights, but you have to take into consideration the fights where you wait for everything to start before you flash in with your ultimate.

The spell vamp doesn't add onto your over all team fight survivability like a Zhonya's does in the long run.

i would convert zyhonas into hextech gun / ancients.
Not bad guide

I understand what you're saying, I understand it 'helps' technically, but getting +10 more hp per tick or however much it edges out to be doesn't really compare to what you could be picking up instead.

Thanks for posting comments though =P at least someone is reading this **** :D

You would be surprised how much it helps, but I am not saying you should change your build to have spell vamp.

This is true, but in most cases if you're life stealing from someone, you're going to be able to outheal their damage regardless. Sure your ult heals you, but in most cases it just isn't going to be enough to make a difference if you get focused. When you die, you die big and fast as fiddles. You aren't the guy that usually skirts away just barely, it happens, but not exceptionally often, and definitely not because of a lil bit more of spell vamp.

second side note: grabbing spell vamp makes drain even more helpful, and makes your ult heal you as well. SV on fiddle is far from useless. it's like saying that Force of Nature is worthless on Dr. Mundo because it does the same thing as Goes Where He Pleases.

Good guide/explanations, I would add pictures though. It will make your guide look less like a wall of text, and more fun to read. +1

Side note: in order to avoid troll voters, I would set your guide to require comments to vote