this is my first build on mobafier. im a italian boy (sry for the bad eanglish:i lern write in chat of lol xD) and i wont to show u a nice build for the champion of broken finger(r)....
this is a build for all game:normal, ranked,dominion... you can get a good score with easy item, sooooooooooooo enjoy =).
Karthus is a ap mage renge carry, it's a gooooood chempion for all lane:mid,botom,and top,but it's better on mid lane. he can farm a lot of minions with his first ability. if you can't kill the enemy midder, feed you with kill other enmeys lane with requiem.unfortunately kathus havent't got a lot of health and in this build you lern to play thefens but strong:- remember;
the better way for defens is....Attak!- by Elva

I have rune my Karthus in this mode
I get ap for level because your ap incrase for 3 at level so at level 5 you have 33 ap if you havent buy another ap items; in mid game with rabadon's increse your ap for 15 at level +or-.
Seal of vitality increse you healt for 10 at level, mark of insight it's usually for mage increse spell penetration, so other quintessences.

if u wont you can change the spells with u wont :)
teleprt:
for gank,return in lane or defens a turret it's very useful

Ignite:
by fire to your enemies vhen you going to die use requiem and get a lot of kill ;)

Pros
-very big farmer-
-if you dont kill in mid kill with r-
-kill with passive-
-spam q-
-good team_fighter-
cons
-low heal-
-all enemy hate you-

in mid lane you can farm more then a lot of chemp so farm: spam your q and slow the enemy and spam q before him then use r.talk at your tema when u have the ulti for kill in all lane. in team fight stay behind the tank or off tank,so spam q,w,e when the enemy focus you.
when the enemy stay at baron use your ulti when the tank attak enemis.

It's soooooo easy for farm with Kathus:SPAMM q and e
you get more money ehit farm an XP in a first 5 minutes of game you get the level 6...
Now start the fun!! and...... :-KARTHS ULTI!!- MHAHAHAHAHAHAHAHA: triple kill!!!

THE OP KARTHS SKILL
lay waste:very small cd big range and damage: if you strike a lot of enemy the damage is reduce but if you strike 1 he get very big damage for repeatedly
WALL OF PAIN: slow all enemyes enter in this, show the dark parts of map.
DEFILE use for farm minions, samll cd,big damage at mid and late game.
REQUIEM:ultimate,damage at all enemy team,lang cd.
COMBO:use wall of pain for slow enemy and spamm q whe he escape deal damage with e and REQUIEM!!
xD ENJOY!




P.S.: say in chat alwais : ULTI FOR FUN!!

how to solve the problem of the blue
blue is important buff for karths: get mana and cdr but anothers time you go to get it ther an idiot dog for gank you...so use the wall of pain bheind the blu --->so use q,e for kill him and get more mana per sec. when you have spell vamp gent the dragon solo at level 8/9 if the bastard dog haven't get it yet. I think karthus isn't a jungler...no mana,no healt and haven't possibility for gank...alwais REQUIEM for fun xD

VERY IMPORTANT FOR MORE POSSIBILITY YOU CAN WIN THE GAME
You and your friends can gank at min or at blue if theres a jungler.
In mid 3 get the bushes of top and 2 of bot: if the enemy midder wont escape to bot he find a surprise! you get the first win, the enemy is demoralized so you win th game =).
Ugual for the blue: gank two bot and 3 from top your team kill the bastard dog and you are happye xD.
At 5/7 minute in game, buy a ward or vision ward for avoid a lot of gank the Mock werewolf wont to wants to do to you.
REMEMBER
push
farm
steal with ulty
support your team
enjoy =)

if you wont you can chenge the potion with mana potion and the morello's evil tome with deathfier grasp.
In the mid game it's very important get sorceres, will of the ancients and the rabadon's deathcap; in the first phase of game you get more mana with tear of the goddess and if you spamm more at mid game you arrive to get 500/800 mana+.
In late game it's important get more spell penetration, cr and so ap.
If the game it's very long sell your void staff and buy another rabadon's.
QUESTINS
Why spell vamp??
spell vamp it's a very important pillar for Karths: you can stay much time in lane and if a dark wolf would to eat you he will get only pain:open your defile and spamm q; you can heal very mach at yourself.You can go to solodragon at level 8 only if you have spell vamp.
The will of the anciets get ap at all your friends and you get assist if stay near your friends.
Mana is very important at first phase of game: in mid or in lane, for farm or kill enemy you have so much to use mana so if you haven't you can't farm and kill.In late game the archangel staff provide to convert your mana in ap.

THE AND OF THIS BUILD
Hi my summoner name is FraSaVe add me at your friends to play,speak,let us have a laugh together and for more detalies of this build.
so:ruens

mark:magic pen.
seal: health per lvl.
gliph:ap per lvl.
5essences:ap

items:

tear of the goddess+mana or heal pot.
sorcere's
will of the ancience
rabadon
archangel staff
void staff
morello's evil tome or deathfier grasp

masteries:

21/0/9:get the attak ap and mana regeneration movement and time spent dead

Tis build isn't finish but there's the minima information for do a good game with your allied.
I hope this review was satisfactory and that has helped to make some good game.
I repeat, this is my first building for "MOBAFIER". I'll do it again and I hope to learn some way to put more pictures in my comments, expand the way I parlarem, stop insulting the poor turtleneck Warwick. I hope to have a lot of visualization and comments from you

Have fun!