This is my own Karthus build. I tried to do good with maxing and getting kills by ultimate for sure but DON'T KILL STEAL.

Pros:

+ Good farmer
+ Good harrasser
+ Awesome passive
+ Great in teamfights(BIG DAMAGE,CAST AFTER DEATH)
+ Ultimate
+ Hard to gank with and

Cons:

- Slow
- Being first target
- He need items to be good in teamfights and late game
- Easy to gank with cd on ,

OFFENSE :

9 Points
Ability power.
Need for second offense masteries line.
Reducing spell cooldowns.
Good for magic penetration of spells.

Utility :

21 Points
It's good with your reduced cooldown by .
Increase total hp and mana.
Faster exping.
Longer neutral monster buffs(it will be usable at more team fights)
It gives same mana as you gain to your team mates.
Faster moving. You can run more easy.
Reduce cooldown on flash. Can save your life.
Reduce cooldowns on spells.
Reduce recharge time of your summoner spells. Good with .

Greater Mark of Insight

- Magic Penetration for improving your ultimate and spells.


Greater Seal of Vitality

Greater Seal of Vitality

- Good for survivality and team fights.

Greater Glyph of Force

- Improve spells.

Greater Quintessence of Force

- Improve spells.

Spells:

is good for running . If you need combine with .

of course it's for mana i always use it after long Defile using.

OTHER:

if you want use it for pushing and coming to lines... it can help but i prefer clarity,flash.

Lay Waste

-> Creates a delayed blast at Karthus' cursor position. After 0.5 seconds, deals 40/60/80/100/120 (+0.3) magic damage to each nearby enemy, this deals double damage if it hits only a single unit.

Cost 20/26/32/38/44 Mana
Range 875

Use it at enemy location or place where he will move in a moment. IT DOES 2x DAMAGE WHEN IT HIT SINGLE TARGET !

Wall of Pain

-> Creates a 800/900/1000/1100/1200 distance wide wall at target location. Enemy champions that pass through the wall have their armor and magic resistance reduced by 15/20/25/30/35 and movement speed reduced by 40/50/60/70/80% (their move speed slowly returns to normal over the duration). The wall lasts 7 seconds.

Cost 100/100/100/100/100 Mana
Range 1000

Good for running or you can use it at enemy location and then spam (it's good cause wall reduce magic resist) ! Good for discovering hiding enemy !

Defile

-> Toggle Off: When Karthus kills a unit, he restores 20/27/34/41/48 mana.
Toggle On: Drains 30/42/54/66/78 Mana to deal 30/50/70/90/110 (+0.25) magic damage to nearby enemies each second.

Cost 30/42/54/66/78 Mana Per Second
Range 550

At low lvl i'm don't use it so much cause it cost a lot of mana! At lvl 8-9(or when you have enough mana ) start using it in team fights or pushing lines . I use it at tower too when i am with my monsters it's useful thing !

Requiem

-> After channeling for 3 seconds, Karthus deals 250/400/550 (+0.7) magic damage to all enemy champions (regardless of distance).

Cost 150/175/200 Mana
Range 10000

Don't listen to guys that say "Karthus is for noobs." "It's all just about ultimate" they are wrong you can't kill enemy or get them on low hp just by ultimate. Talk with your team if they need it use it... Communication is useful ! Don't use that skill at enemies who can stun you,enemies with shields(invulnerable spells like ), without cd on heal or at low hp !

Death Defied

-> Upon dying, Karthus enters a spirit form allowing him to continue casting spells while dead for 8 seconds.

No cost

After you die cast ,cast (if you can) and of course spam , If needed than use .
Usable in TEAM FIGHTS you can help after death !

Enemy MAGIC RESIST

START:

First Items :

Mid & End Game :

Enemy Magic DAMAGE

START:

First Items :

Mid & End Game :

Enemy Attack Damage

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First Items :

Mid & End Game :

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I am using this item because it have got magic penetration for my spells,ultimate. Spells,ultimate does more damage.
Gives mana bonus and converts 3% of max mana to ability power.
That stack up is useful try to get it as fast as you can for maximum stack ups !
Ability power at this item is really big and good for .
Useful slow.
Magic Penetration is good for spells,ultimate that means you deal more damage by spells.
Gives you magic resist vs. casters and reduces their magic resist.

Very good at pushing,farming,killing monsters i don't use often on monsters better for it is . REMEMBER TO TURN IT OFF IT COST MUCH MANA !

1. Don't go in team fights as first !
2. Don't use ultimate when enemy can cancel it or kill you !
3. Get buffs if it is possible !
4. After death try to help team or kill someone !
5. Defile take much mana don't have it toggle on still !
6. Ask your allies to help point out when you should use Requiem to get kills in different lanes.
7. Lay waste is very strong at farming minions and harassing enemy champions.
8. Don't fight with in same line. You're stacking his spell. It's impossible.
9. It's hard to fight vs , it's hard as with so beware being in same line.

If you are top,bot without team mate cause he is afk,jungler,leaver then don't be in same line with this champions they will don't let you exp :
+Any champ.
+Any champ.
+Any champ.
+
+ -> that heal is annoying :/
+ or
+ or
I tried it and it's really hard to play vs those champs. all the time they was bigger lvl than i and they killed me easily.

At top/bot line good team mates for you are these :
, , , , ,etc.

If you like my build rate it and comment what could change,etc.
I'm Playing on North America "deathbringer77".