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Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Well, this is my first build on mobafire, and to be honest, I think it's a unique build. I definitely won't say it's the greatest build in the world, but I find it the most fun. This build is not for players who are beginners with karthus; this build makes him probably the most vulnerable champ on your team.
The title pretty much says it all; this itemization of karthus is extremely one sided. You will either do extremely well with him, or extremely poorly. Either way, I think you'll have a lot of fun with it.
The rune set I have provided is actually the rune set I will eventually play test. I don't actually use these runes right now (I use ability power/level marks and glyphs, straight up ability power seals, and magic pen quints), but I would like to eventually give these a shot.
Now, for explanations of each:
Because of the itemization of this build, we will NOT be buying any items with magic pen, which means all of our magic pen will be from our masteries and runes. These are practically a must.
Because we are still trying to be as one-sided as possible, I chose the solid ability power runes just because in the early game we want to be as threatening/harassing as possible. This solid buff to ap will make our lay waste hit even harder. I also chose solid ability power because of the summoner skills I take, and the fact that we take lay waste and defile at practically a 1::1 basis, so we won't need mana regen, and the items we take allow us to not need mana from our runes.
Cooldown reduction is not to be underestimated, though these runes are probably our most flexible ones to fiddle with. If you don't think you need the cooldown reduction (or just don't have them yet, as is my case), then feel free to experiment with different ones. If you think you won't have enough magic pen, you can grab them, or if you want even more ap, take either of those. Either way, go with whatever direction you feel, but the cd reduction I should think would be the best.
As stated above, all of our magic pen is through our runes and masteries, so we're going to want as much magic pen as possible.
So, with these runes, we have more ap, cd reduction, and lots of magic pen. With the masteries and runes, it should come out to close to 30% magic pen right off the bat. That should be about all we need.
I've toyed with going a full offensive build, but then we lose out on a lot of really good skills in the utility tree, and frankly, having as much mana and experience gain possible is a must, so going full utility is a must. The only reason for going full offense would be for the last ability; nothing else past the magic pen is worth it for karthus.
The skill sequence for this build is (I think) rather unique. We want to go as offensive as possible, so we won't be taking the wall until level 13. Yeah, that's quite a ways off, but it allows us to stay in our lane for a lot longer than anticipated (specially if you take the summoner spells I do). If you have a particularly aggressive mid, say, Sivir for instance, then I usually just tower hug, but feel free to substitute one level of wall for any of the skills (except requiem, of course). I think of it as a priority system (like a ret pally's rotation in WoW xD); Requiem is out top priority, so if that's available to take, do it. Next up is Lay Waste, so if you can, take that. Afterwards is our Defile, so if the other two aren't available, take that. And finally, if nothing else is there, take the Wall.
The Summoner Spells I take with this build is almost always Heal and Clarity. A lot of people say that a good karthus doesn't need clarity because of defile, but spamming our q just to keep the enemy away isn't last hitting minions, so we won't get as much mana from it as everyone thinks we will. Early game, clarity allows us to stay in our lane until we have enough to buy an archangel's staff straight up. I almost never leave mid beforehand.
A couple of other considerations would definitely be Flash. By far, the best escape mechanism LoL has to offer, specially for a champ who doesn't have any CCs, and definitely the only option to take for this build before level 13. I prefer the heal, but I can definitely understand someone taking flash and clarity, or even heal and flash with this build.
Exhaust is another really good option for Karthus, and can even be taken in the masteries in place of the crit chance. For a character without any CC, and not taking wall until 13, exhaust is another one of those 'oh ****' buttons, even with the changes to it. Also, it can be used to catch those that you don't want to use your ult on.
Now, for our item sequence. I've never made a build on this site before, so I'm not sure if it'll show the order in which to take items in above, so I'll summarize here as well. I always take the mana sapphire at the beginning, and then a health pot and a mana pot. This will allow us to stay mid longer than your opponent; this is almost always the case, specially with heal and clarity, and it is important that we not return until we have enough for that first archangel's staff. If we do have to return, then I would go with the tears first, then the blasting wand. After you have assembled the staff, it's time to go for our hat (rabadon's deathcap I think it's called? This site doesn't have zhonya's split up yet).
After the hat, I will always go for movement speed, which means boots. Before the patch that introduced the cd boots, I always grabbed boots of mobility. Boots of mobility, you ask? Why would we need those if karthus can use his ult anywhere on the map and still effect gameplay? Because it's still an extremely important factor to be present in team fights, or getting to that minion wave to farm gold. Plus, those boots may be the key to chasing down that champ for the lay waste kill...enhanced movement speed of 5 is pretty hard to escape from. I now, however, almost always take the cd reduction boots, but that may also have something to do with the fact that I don't use cd reduction runes. But, feel free to take either.
After you have your boots, hat, and staff, it's time to start stacking! Grab any mats you feel fit for more archangel's, as long as you only go for them next. If the game lasts any longer than 40 minutes, you should have your initial staff, your hat, your boots, and then at least two more staves. People will wince when you have over 1000 ap, and your ult will start to hit for almost half of your enemy's health total. If you feel so inclined (as I usually do), and if the game goes on for this long, you can sell your boots for another archangel. I have reached 1438 ap doing this, and the ult will hit for 1557 (550 + 1007). This will surprise the ever-loving **** out of your enemies when you use your ult when they're only at half life, and kill them anyways. LOL
Now, even though we're squishy as all get out with this build, when it gets past the laning phase, you want to be at the front of the fight, because our defile will tick for ridiculous amounts of damage, which will not only help out team out, but allow us to pick off running champs with our lay waste (if we're in back, we won't be able to reach them). Plus, because of our passive, if you do happen to die, you can keep your defile on and peg champs with lay waste, even when you're dead. And, I almost save my ult for when I'm dead, just so I can be sure to finish off that champ that was silly enough to get CC'd in my defile when I'm dead.
To sum this all up, this build is not for the feint of heart. While I find this build very fun, you really have to be careful with it. While we will be hitting extremely hard at all points of the game, we do not have ANY defenses (aside from our summoner spells and team mates). Like was stated earlier, you will either do fantastic and make people cry, or weep deeply yourself. I normally get 20+ kills with this build, though most of the time, my deaths are almost as high, but that's because I'm ALWAYS focus-fired in team fights, which is fine with me, since the passive is always there and we almost always win that team fight.
Do keep in mind that this build is truly meant to have fun with, and I don't expect it to be the best, but just an option for any experienced players out there to toy around with. Any suggestions or constructive criticisms are always welcome.
Oh, and one last thing. Please don't try to tell me that stacking deathcaps will give more ap than stacking staves. I did the math; the caps fall about 150 ap short of the staves, and we want as much as possible. Plus, the extra mana is extremely useful. I usually cap out at around 4436 I think.