Build Guide by Mulishaman
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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, the mana *****.
Key part to this build is being mid or solo (hopefully not 2v1). You will want to make sure you farm to the best of your ability, if done well, you could have this full build before the other team or your team are half way done with theirs. Another thing to remember, you are squishy dont play like you have lifesteal or your a 6k hp cho'gath!
With Karthus, team communication is key. You can become "fed" off of just your ult.
I start with sapphire crystal because of the AP and MP you get from runes will be enough to get a kill or two fighting mid 1v1. You can also substitute that for a tome and go for mejai's first if their team isnt heavy on CC and you know your mid opponent is squishy (teemo, twitch, ashe, etc).
To harras your opponent a little, aim your wall of pain correctly, slow them, then lay your lay waste in the path they will be running, with this combo, the slow will make it so the only way to avoid your lay waste is for them to flash, or something ghost. This is cause them to either back or become cocky. Continue to do this, when you reach level 6, aim to have them, already at half hp, or around that. Lay wall of pain, lay waste, lay waste, then flash in with ur defile, lay waste, then start to return to your tower. If done right you will have them low enough to pop your ult and kill them (if your lucky your team will have a opponent or 2 low and maybe you will pull off a simple double kill.)
After your kill at lvl 6, go shop. You will now start to gank, top/bot, then up or down to the lane u werent currently in. Assists mean more then people think. Once you have ganked both lanes, return to mid. By this point you should still be even lvl with your solo opponent but be out gearing him. Continue to use the wall of pain, lay waste, lay waste, defile, lay waste strat and you should get some kills.
Once late game comes, if you havent died constantly, you should get your full build before anyone else. This is when the fun starts. As i mentioned at the top, this build is not for the weak hearted, you are extremely!!!! squishy, but you will hit like a truck. Against anyone that doesnt have a force (excluding tanks) your ult will hit for 1.2k+ (i have hit a twitch for 1.4k before). Your ability power should be sitting at 1k-1.1k. Once you have your full build, feel free to purchase HP and AP pots, a lil extra HP and AP helps. Your lay waste should be hitting single targets for about 800-900 dmg, enough to kill someone in about 3 hits. Just remember you are squishy, you stay back, you stay in the bushes, do what you have to do to become the last targeted. I like to hide, and flash in once a fight starts, using my defile as a sort of Fiddlesticks ult, and then just blow things up.
Thank you for reading, note this is my second build on this site, hope you enjoy, and have fun!