Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Karthus - [Apocalypto]
There's not much to say, most of the guys who read this build, know Karthus'
abilities and his passiv. So just buy and skill how this guid says and your team wins, if you dont fail to hard in middle. Your teamm8s should let you some kills / enemies with low health / so your Soulstealer gets stacks. You should use your Q spell against minions, so you get your mana back (passive skill). For good damage, try to attack your enemy champion solo with your Q spell, so that your enemy gets only hit (double damage if only one enemy gets hit with your Q spell). At the end this will do about 600 Damage only with your Q spell.
How to attack best is easy, place your slowing wall behind your enemy, spell your Q skill and then, if your nemy is near, use your E spell to do some awesome damage.
How to use his ultimate: Must I say something to that? Look at your enemies, (have they low healt??) - yes? hmm. You would say press Ulti and hope that you get him.
BUT: Care, some of your enemies could have a shield, like Sivir or they could have Zonyas Ring. Of course, Guardian Angel is very useful against your ulti. But most of the time your enemies won't play that long time to get that ;).
Of course you are able to choose your own Summoner spells, but these are definitly the best for middle. Clarity is a must have with Karthus because of his low mana pool. I would prefer Clarity and Blitz for Top and Bottom, so you can reach your enemies if they are to far away from you.
Once you have truly experienced Karthus at his true potential you won't go back! He is ultimately the king of the battlefield and is not to be taken lightly. If you are having trouble with him in a 5v5 try a 3v3 and just see how easily you get that triple kill. Once you've mastered annihilating an entire team in 20 minutes flat on a 3v3 you should have no problem holding your own lane plus your neighbors in a 5v5. Best combo that works well with Karthus lane wise is someone who can stun slow and chase down. (Ashe or Taric are my two favorite lane partners.) The Blue Golem buff is not crucial to have the game so don't get discouraged if you don't get it. Only thing the blue buff is good for is quicker wall of pains or Requiem. But the cooldown on those are rather fast.
Commentary Level 1-18 (Written by Diggifreak045)
Beginning of the game I choose Sapphire Crystal and take a single lane. From here just last-shoot minions and harass the enemy champ(s) with laywaste. As soon as you have the money to get Tears of the Goddess grab it! Getting it in under 5 minutes into the game and you won't have to make any mana run's mid game. After this we should have Amplifying Tome and getting close to mejai.
By this time we have requiem and can do a bit of damage... But not quite ready to take on the world! From this point keep pushing your lane slowly going for ganks, buying mejai along the way and keeping an eye on the map for free points in mejai! Our biggest step next is becoming not so "squishy" so as soon as you get that 1110 gold recall and buy Giants belt!! At this point you should be around level 8 -9. Keep an eye on the jungle trying to get all the free gold you can By level 11 you should be finished with rylai and starting Lichbane for some speed.
With Wall of Pain and Rylai keeping his enemy's at his speed and practically defenseless to a barrage of nukes landing infront of them it time for the big guns, LICHBANE!~ Slowly buying the parts Lichbane is going to start paying off dealing massive not just magic damage but Attack damage now. As soon as you can finish off Archangel's staff for a nice little Ap Boost. Between Wall of Pain, Defile, Laywaste and Your regular attack Karthus is now able to do about 650/second with magic and another 400ish with his regular attack. Depending on magic resistance and there health most within Karthus range is not getting away...
By this time depending on deaths and just not getting that gold finish off all previous items listed and prepare start on Void staff. The extra Ability Power and Magic Penetration will truly devastate all who come within range! Once this is achieved Karthus can start on his big weapon Zhonya's Ring. Chances are you won't get Zhonya's Ring... They will either have surrendered or you will have clinched your teams victory all the way to the nexus with being able to 3 hit turrets with lichbane. In the off-chance they have held on, they're in for a BIG surprise! With nearly 1000 Ability Power and being able to 2 hit Squishy champions This is Karthus's Battlefield. Stick with your team to prevent the unfortunate gank. Karthus can take on 2-3 by himself and most-likely live, but will not be able to take all 5 (at least not yet ;)) At this stage in the game, try and get blue pretty often as if you have Mejai's stacks maxed having Requiem under 1 minute is just amazing!! Just keep pushing, and making an example of those "tanks" that think they can 1v1 You!
The world is yours. You have won the jackpot! :)
Keep playing Karthus so you'll get better and better.