Ive been playing Karth for a bit now and i come across the same problem every game, What do i buy....... I have no trouble getting the all important farm and usually pick up first blood. My problem is when i start purchasing core items. I usually go for what i can afford. i think this is my problem and i have a hard late game. I do know that playing an ap carry can be very situational especially if the enemy team builds MR, what i want is some core items that would never change no matter what the other team is building. An opinions would be helpful

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im not a ''pro'' but karthus used to be my main champ (hiemerdinger now). But i always found doing a pure ap build gave me the best results. ''NOTE'' i dont play ranked i only play normal so this may not work in ranked matches. Id start with a mana stone and 2 pots, then upgrade to a tear of the goddess and buy lvl 1 boots (2 if i got first blood and good farm).then i upgraded to a AA staff, next i get a deathcap, followed by 3 more aa staffs.(you should have a little over 1.1k ability power 1400 ability power if u ditch the boots for another staff again i dont want to survive the teamfight)basically at the start of a teamfight i just charged in turn on the aoe , put up a wall on top of them and spam Q as much as possible before i die, once im in my passive i ult then continue to spam Q usually picking up a double or triple kill( depending on if there team had alot of squishys) this build is not for everyone but i always had amazing success with it.i use the same build on hiemer but just use a different playstyle.All of this was before the nerf on karthuses cooldown for his ult so now i am forced to get cooldown boots and a athenas and a cooldown potion so my dmg has gone down alot but i still get decent results.

When u play karthus mana should never be a problem do to his e's passive. Basically put one point in ur q to start and try to max ur e fast.If ur good at last hitting mana won't be a problem. I like to build a bit of a tanky ap karthus in ranked .Start off with boots three hp pot .Get mr pen boots rush roa, death cap rylais, zhoyas

Item build order:

Start boots/3 pots
Rush RoA (tear is a noob trap imo)
Sorc boots
Deathcap
Zhonya's
Void Staff (if they're building even a little bit of MR, it's good)
Final item depends on the game really. WotA can be nice, but really it's situational.

Also, never max E first. Skill maxing priority is R -> Q -> E -> W

RoA takes too long to become useful. I rather build Zoid Staff then proceed with DC and what not.

Starting item, Meki pendant. No pots. A good Karthus doesn't need pots. You shouldn't be trading early, and you shouldn't run out of mana once your E is rolling. Upgrade asap to Tear. Get boots. Once tear can be upgraded to AA, do so. Somewhere before AA is built you'll have enough for your sorc boots, get em. Then the rest of the game just get the following items. Rabs, Void Staff, last two are situational. Zhonyas, Rylais, etc. .Just depends on who you're facing and how things are going. The key items every Karth should have though, are AA, Rabs, and Sorc shoes. I get a void staff only if the other team is building MR, otherwise I let the shoes be enough to cut through.

start with 3hp and boots.
build a tear of the goddess and build into arch angels staff. Then proceed to build into ROA and getting sorceress boots. proceed to get items like death cap,abyssal scepter,zhoyna's hourglass, void staff and ryleias crystal scepter.( anything go's really just remember the key items are archangel's staff and ROA). don't have to build them all but these items are all good late game.

Remember your job as karthus late game is to just pop ur e and spam q,w. and try and postion the most of the fight within your aoe. once u die pop ur R. Your job is done.

People.I really recommend dorans ring it may not be core item but its help is extrem pen boots+ 2 dorans and 80 ap giving rod ur damage is already great! Grab hour glass and death cap gg