/!\ This is particular Karthus' builds. The masteries and runes are required to make it efficient. This Karthus build aim a GREAT survavibility in team fight for more AOE damages and soft controle. /!\

Test it before to vote. I made a lot of games with it and it is really efficient.

Whyand?
You need ghost to escape / Chase. I think it's better than flash cause you can use you're 'E' to damage while you're running and you can't with flash.
If i take clarity it is cause mana will REALLY be a prob in start game and start game is you're MOST IMPORTANT time in the game. You must feed yourself quickly! It's also great for you're carries in end game (Think about Irelia making penta and asking for mana cause the poor child spammed too much!)
You Can also choice Exhaust / Flash / Ignite But nothing else

As you know karthus is a mid champ.
This build is MADE FOR MID don't try in lane (solo top maybe bot not in a couple). The reason is that my build is a "tanky" way of karthus in the first time. If you take any ****** in exp you're skills won't make the requiered damages to go with items and you won't kill so won't attempt to second item part who's based on damage and then you're game will be ruined.

Then you're a karthus mid. You have to know that your main damage source in lane phase is your 'Q'As you shall know it makes 200% damages on a single target. If you're opponent is peaceful and just farm creeps try to make more than him and hit him in second. If he/you want to be more agressive, and you can!, just wait lv 4 then run on him withand place the wall behind him. You can be sure that he'll go back, then in the wall so you're 'E' will make lot of damage and the 'Q' mines that you just have to place 'Q' mines just front of him and he'll walk on it (and die :D). REMEMBER: You're not LeBlanc. Don' leave your lane to gank. Push if you killed ennemy champ and use your ulti if it's needed. Never loose exp, that's the most important!

/!\ Don't worry about diying if you're SURE to take the kill. you don't have snowball items but you really need fast gold to rush Rod Of Ages and start rule the lane. /!\

In teamfight you shall have around 3K HP and 150 AP (with your two first items) that's time for you to get feed! you shall have more HP than any opponent (even tank) with huge damages that makes easy kills.
=> You're focused? Place your wall on the ennemies, rush them with 'E' and spam 'Q'. If you don't get the kill and you die you can be sure to get a lot of assist from your teamates who have ennemies deadbody everywhere (rush ennemies is important to have the bennefit of the passive)
=> You're not focused? Place your wall on the carries / dangerous ennemies and... Rush them kill them feed and GG :D!

Tips: Your wall is a great snare but huge CD(and mana cost). Don't waste it be sure that it will be usefull.
if they are running on you (creeps / heros) place a 'Q' mine behind you quickly and run again to snare them. You can place a mine run place a mine run place a mine run... That's an insane harrass!

HOW TO USE MY ULTI?
Karthus' ulti is know to make ennemies rage.
It has 3s cast. If you're silenced / stunned / Bumped / killed while casting you won't launch it!
You have to use it to last hit ennemies. use it at the beginning to suicide on ennemy and get the kill. Use it in teamfight after a suicide inside ennemy team to deal even more AOE damages. use it from your base to snare and ennemy chasing your allies (with Rylai).
BUT Take care of: shield / Spell blocker / black shield / Heal (soraka / invoc. Spell) / great life steal ennemies / Item (Zhonya / hexdrinker...) / Shaco's ulti (if nicely timed you avoid ulti when you divide with clone).

=> Don't hesitate to rush ennemies. Even in 3v1 if you're SURE to kill one of them go on you need gold fast at the beginning!

The Runes aim a great armor / Magic Def to stay in lane longer (so farm more) but also to survive a gank/ have first blood easier. I really like to start with a lot of armor / MD. Ennemies think Karth is squichy so they rush and they can't get out without diying.

The masteries give you even more tanky start. More HP, More armor / MD and 4% réduc damages. But this is also a great end game tree. You have % of mana in regen HP (really great in long team fight) and +4% AP that makes a little help :)

The items are the main part of this build. In effect i prefere a tanky start with hybrid HP/ AP items to farm a lot of creeps and to get easy kills on ennemies mid champ. For exemple you mid a LeBlanc, you can be sure that she will be hard to assassinate or get you out of the creeps. If she tries to kill you yourwill damage her even silence and yourwill keep her in respect (or kill her if she goes on) before she can kill you or escape.
With the 'kamikazi karthus' you'll get kills for sure meaning items quickly and HP coming fast. Even if you die while making your 3 first kills (for exemple) you'll get a great advantage with those items and start to kill without diying. You'll be able to do tons of damages in teamfight without diying.

The 4 first items are really important and the order is also important to have a great HP / mana / AP pool + Snare. But the 2 last can be adapted to every situation. Here i placed armor / MR items to be even more resistant with AP and lovely effect (2s invulnerability with 'defile = <3 / -20 magic pene = lovely carries come here) But if you're doing well Rabbadon / Ancient will / lich bane / another archangel staff is acceptable.

I'm sure you're thinking: "A Mage without rabbadon?" yeah it's strange but you have to know that even without it i go on 500AP. I loose like 300AP to get precious second in teamfight. Longer you stay in team fight less they have chance to survive. Karthus burst is impossible to tank or avoid (AOE) so the seconds won will REALLY make the difference.

In conclusion the items must focus first: couple of HP and AP then AP/ resist if teamfight are heavy damages overall OR AP burst (ancient will is ftw in this case). Depend on the game, the ennemies, your allies and what is needed.

Cons:
-If you fail a kill your time will think you're noob. (cause suicide risk :/)
-IF you fail you start game you'll have hardtime with fed mid !
-You won't make an insane 'burst'. Global damages are really better but don't think you're ulti will make a penta and GG.

Pros:
- You never farmed like this!
- Tanky = less successful gank + Stop take 2 Ashe's arrow and down in team fight.
- Can win 2/3v1 if good start.
- Usefull even in end game.
- Nice ulti + Great snare and insane AOE
- You can jungle with this build!! Start with heavy pull to the blue and

In my build i try to make a tanky Karthus cause i think that survive more = more time to damage = better in time fight, better to farm and easier to kill in lane/ Take ***a downs.

Don't forget to vote if you like it :D!

If you think there are things to change just advice me and i'll think about it!

Enjoy playing Karthus and make whole team rage <3!
(as you can see i die a lot but i push well, i ruin the ennemy middle and i get my stuff really quickly so i help a lot in teamfight!)