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Choose Champion Build:
Since you are probably midding against a magical damage based champion, get Chalice of Harmony first. It is an awesome item - it gives you 30 magic resistance, massive mana regeneration and it's cheap. It is this item that can allow you to zone your opponent with bombs, or farm minions with them if needed. And it is also this item that will help you survive the enemy magic burst. Get it.
Now the second item is where it gets fun and diverse. If you lane against a heavy burst champion like Veigar, LeBlanc or Malzahar, get a Giant's Belt to survive their combo. It might seem like a stupid item, but along with Chalice of Harmony, it is a direct counter to enemy burst champions.
If you feel comfortable against, say, Heimerdinger, get a philosopher's stone instead. It will regenerate your health, give you even more mana, and generate gold overtime to pay for itself. If you don't like this item, skip it and continue in the build, it's completely optional.
Now is the time to get Boots and then rush massive ap. Well, then why the hell do i have Will of the Ancients in the build, when massive AP is Rabadon's Deathcap? If we do the math and take item cost into consideration, Will of the Ancients gives only a bit less ability power compared to Rabadon's Deathcap; assuming those are our first AP items (which they are). Now Will of the Ancients give you a lot of spellvamp, which is extremly strong on karthus early on. The bombs deal massive damage and the spellvamp will help with the sustain and even grant you insane survivability when you fight for your life. Again, the later you get this, the less effective it will be, since other people will start getting insane damage, being able to outDPS your spellvamp easy.
Next continue with Rabadon's Deathcap. There is no say, you need the AP. If you start dying now, forget about getting Giant's Belt, the times when it helped you are long gone, now you actually need damage. If you chicken out now and start getting survivability, it will suck, because you will lack damage -> get no farm -> have even less damage -> be utterly useless. On the other hand, if you get pure damage, then even if you die in every single battle, your passive will make sure that you are able to use the damage you bought and you will be usefull and eventually get to the survivability items and stop dying again.
Get Rylai's Crystal Scepter, unless you aready have it. Then get a Void Staff for even more damage or Zhonya's Hourglass if you get tired from using your passive all the time.
If you get awfully rich, swap Chalice of Harmony for Archangel's Staff if you feel like it.
Why? It gives you regen, but not enough, it gives you mana pool, which you really don't need unless you tend to forget your Defile on, and it gives you HP, which you can get by other means or not at all. If you want to build bulky, get Rylai's Crystal Scepter instead - the slow really helps a lot.
Now before you start flaming me about this, i say yes, Rod of Ages is a viable item, but only for high-elo play. I will cover the reasons why it sucks in lower- to mid-elo games later in the champter "Now why not RoA? Solves everything, right?".
When you die, you cannot autoattack. I hope all is understood now.
You have your Wall of Pain and Void Staff, ergo you do not need this item. If you need more magic resistance than Chalice of Harmony, buy a Banshee's Veil after you get your Rabadon's Deathcap. It has a passive that you can actually use.
I am talking about taking this item fast (1st, 2nd item). Now it might look fancy to walk around with your Defile always on, but that's about all this item gives you. For it's price, it gives low ability power combined with massive mana regeneration. But you will be squishy and your ability power will be rather low. The advantage of rushing Archangel's Staff is scaling into late-game. If you manage to survive with this item, getting Rabadon's Deathcap will give you insane ability power, since your AA passive will have time to grow your mana pool. But i do not recommend getting this item early on.
This is why you should skip the philosopher's stone unless you need it, and maybe even skip Will of the Ancients if you can. Do anything you can, cut any items you don't need to get your ability power up as fast as possible while not screwing up your lane.
Now that noob just told us to rush AP and yet he builds Chalice of Harmony! Remember, to get items, you need to farm. To farm, you need mana - either to kill creeps from range or to keep the enemy champion at bay. If you get enough mana regen, you can use your bombs only to keep the enemy at bay and farm with your autoattacks (yes, karthus has an autoattack).
Also, getting a Giant's Belt will actually help you get your AP faster against, say, Malzahar. It's pretty damn simple - if you don't get it, he will kill you. And when dead, you cannot farm (for long).
Now Rod of Ages is a valid way to play karthus, but: it's dreadfully boring and it has one problem. What happens when you screw up, get ganked and your tower gets pushed while you only have RoA? You are screwed, that's what happens. You have no items at all, just the rod. Farming is hard because you have to go out in the open to do so and miss the cover of your tower. And suddenly, when caught farming in the open by the enemy, a bit of extra HP doesn't mean a thing. Your ability power is not that hot, your resists are non existant, Rod of Ages passive useless.
This is why i prefer to get something else. If you are high-elo player, confident that you won't screw up, get the rod. If you are just a normal mortal, avoid it, because Rod of Ages is only good in case you stay in lane long enough to buy Rabadon's Deathcap with it. If your laning ends before you have those two items, rod was a bad choise.
When farming minions, try planting the bomb so it will hit only a single minion. He will then get double damage, allowing you to farm more accuratly.
Wall of Pain
- Use it to slow enemies that throw stuff at you and run away. Zilean
- Always throw it on melee champions that jump you before hitting them. It reduces their magic resistance. A lot. Katarina, Master Yi
- Activate when desperate for damage. Over team fights, don't leave this thing on, since it consumes a lot of mana, and this build is regen-based, not mana pool based. Your mana will not last long if you forget it turned on. Use it only when really needed. Remember that it's always better to lay a bomb than have 1 more second of some AoE.
- Use it to aid lasthitting creeps. And i mean lasthitting, not turning it on and waiting until everything dies! If you lasthit with bombs and see there is a minion with 20 hp that is about to be killed by your caster minions, turn Defile on and off again to get the creep kill. But.. forget this if you don't know what you're doing, it't just a nuance and really not that important.
- Ult to kill! (no sh**, Sherlock!)
- Ult to scare someone away (avoid it as much as you can, but it might be good to scare the enemy from diving your bot lane)
- Ult to help in fight. That means, you ult before everyone get's low on HP. You don't get any kills from this, but in certain situations, it might make your teammates win the fight, which is always good. And you get the assists.
- Don't ult, when you are near a disable.
- Don't ult if you are nearly dead and near combat.
- Don't ult when pissed off that you just died.
- Don't ult a Shaco that has his ult, Morgana that has her shield, Fizz that.. well fizz is just silly. You get my point.
1) You can flash away, saving your butt in laning phase.
2) You can flash in the enemy team when being focused and de-facto force the enemy to fight near your ghost or abandon the position your team is attacking. When you are defending this usually doesn't work that well cause it's hard to prevent the enemy team from retreating. But it is very useful, using your corpse well may buy your team a ticket back to an almost lost game.
Ignite. Now why not Exhaust? Simple, it increases your AP and AD by a bit when on cooldown, it has a decent damage output when in tense situations. And it will travel with the stupid Katarina whether she shumpos away or not. And also, the enemy usually has Ignite, which would give him a huge damage edge over you if you picked anything else.
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