Long ago, people told jokes of Karthus players who built tank. They taunted and mocked him for being "Squishy as Fresh Play-Doh" and "So slow he almost moved backwards". But no more. Behold, Tank Karthus, the savior of ****py teams. As Tank Karth you can do the Karthy things that make Karthus the annoying turdburglar that we all know and love, for EVEN LONGER. Yeah, you won't be able to one-shot people with your ult, or Kill with with 2 Q's, but if you play Tank Karthus right, You can and will win teamfights. ALL OF THEM. Moreover, it allows you to salvage games from the depths, as your Ult allows you to essentially gank without even being present, and your annoying ability to survive and even walk away from fights will leave the enemies crying as you drink their tears with joy.

(*See Core Build for context.) These are the items you will need to begin doing the Karthy Things. Including but not limited to: Sniping Scrubs from across the map for free G's with the Almighty Karthy Kannon, Diving turrets and teams alike and getting kills before walking away, Surviving Ganks while melting the enemy jungle and midlaner before spamming your laugh and walking away, and headbanging to Pentakill music while building up stacks in your base and waiting for an opportune time for a Karthy Kannon. There's 3 types of Karthies in this world. The passive, KSing Karth that everyone hates, who builds straight AP and CDR and trolls in base all game, mashing the R key with Unrelenting pleasure while your team does things. There's the Poke/Zone/Plain Karthy who is exactly what everyone expects when they see you in the loading screen. Plain and vanilla, he does what everyone expects Karthy to do- Pokes with his Q for disgusting damage, uses his Defile if you engage on him, Ults when you escape with low health and dies in an instant if you so much as fart in his general direction. Blech. Boring. And then there's you- the Tank. Early on, you will start similar to the Traditional Karth- Farming with your Q and E, scoring double points if you hit the enemy laner alone and ulting whenever you know it will kill. But remember not to be overzealous in your desire for kills. There are rules that come with the godsmiting Karthy Kannon. Rule #1- NEVER. EVER. ULT WHILE VULNERABLE. Just no. I've seen so many Karthus players who see an opportune chance to smite someone in bot/top, and then rage when they unsurprisingly get killed/stunned by their opposing laner before they can finish. Don't be that guy. Just don't. Unless you're either under/behind turret, or dead, NEVER EVER PRESS THAT BUTTON. Rule# 2- Never ever ult unless you know it will make a difference. This means look where you're aiming. That is 550 damage or more that will be fired with the force of an atomic bomb that cannot be canceled or refunded. that means someone, somewhere, is going to have a very very bad day in 3 seconds. Unless you noob it up by ulting someone with 50%~ health, or trying to kill someone with your ult only to be reminded that they have a shield built into their kit, or that they're running Summoner Heal, Barrier or Zhonya's. Look First, then Shoot. Rule# 3- When you see that you can get a kill in bot lane with the Kannon, Think first about whether or not you can put it to better use. Remember, unless you build a lot of CDR, that Godforsaken ult is on a 180 second cooldown which means that while you might get this kill, you won't be able to get any triples, quadras or pentas for the next 3 minutes, even if the opportunity arises. Rule #4- When Firing the Karthy Kannon, you must ALWAYS, type into the /all chat a parting message. Whether it's a simple "You are the weakest link. Goodbye." or a "ALL POWER TO MAIN THRUSTERS. SYSTEMS ARE A GO. FIRE THE KARTHY KANNON!!" and raise your arms with pride with our lord and savior Karthus, as he brings the Word of Death to all and smites the scrubby infidels with his almighty ult and relish their demise with pride before remembering all you did was press one button. I mean, after all you can't do anything for the next 3 seconds anyway, you might as well.

At full build, you should have a number of things available. You have a lot more resources than your average Karth so remember to use all of them. As Karthus, when you bring home and unwrap your new Zhonya's, you are officially welcomed into the KKK. That's the Kool Kids Krew you racist ***. Anyway, your teamfight strategy goes in this order.
1. Wait until the enemy team is grouped up. It does not matter where, but preferably in a tightly enclosed area where they feel safe or will be reluctant to leave. I'm talking about Baron, Dragon and during a Turret seige. If you let them see your entire team marching down mid lane, they will almost always group up in the safest place they know of to defend it- under turret.
2. Wall them in with the Wall of Pain. You can either throw it directly on them if you think you can, or alternatively, use it to cut off their escape routes if you think they will run. Remember at full level it reduces their MR by 15%, and slows by 80%, so they will be hesitant to walk through it, especially if they presume that you're a squishy and easily dealt with.
3. Turn on the Team Melter and Flash in. When they see a lone, Squishy Karthy walk up to 5 people and potentially a turret, 9/10 people will perceive you as a fool and instantly get tunnel vision, jumping on what appears to be a free kill slathered in salt and ketchup. However, if you were a good Karthy and remembered to bring all your toys to the fight, you will have multiple Barriers to soften the blows. You have Summoner's Barrier, Seraph's Barrier, and, if you're feeling saucy, a third barrier from Locket of the Iron Solari. This buys you time as you melt the entire enemy team and they blow all their spells on a tanky Karthus who just doesn't care as almost 1000 points of their damage is negated.
4. Should they break your barriers or they wear off, whichever happens first, They will still be focused on the Team-melting Karthus that is trying to hug them. At this point, they can no longer ignore you as you do too much damage, and will be reluctant to leave either because your wall will slow them or they're defending an objective, and will be forced to dive on you. This is when you Zhonya's and laugh for 2 seconds as you continue to melt them all even more.
5. Die. Not for realsies though, as your Guardian Angel will give you 4 seconds of stasis to laugh at them some more as your team rapidly approaches them. This is the point of no return where you decide if you want to pack up your toys and go, or stay and play. This is also the part where your team can save you and drop their Heavy AOE CC. I'm talking Sejuani and Amumu Ults, I'm talking Orianna and Yasuo combos, I'm talking about Katarina Cleanup and all that jazz.
6. Die Die. Like, for realsies. Your Job as Karthus is to die. Now there's a distinction to be made between dying and feeding. You want to die in the place that will be most beneficial to your team as Karthus relies heavily on location based combat. Remember that when you die, your passive grants you another 7 more seconds of Team melting agony. That is also when you pop your ult assuming there's anyone left to kill. Now let's do the math. At Full build, Tank Karthus rocks about 400-500 AP. That means your Defile does approximately 200+ damage per second, plush a % of their current health because of Liandry's. Your barriers should buy you at least 1 second, your health and innate tankiness should grant you another second or two, and your zhonya's and passive grant you another 2 and 7 seconds respectively. That's a total of 11 seconds of Nauseating damage, plus the Karthy Kannon at the end for another 850+ damage, which means at the end of a teamfight, you and you alone will do a minimum of 3050 damage TO THE ENTIRE ENEMY TEAM. Assuming you followed these directions of course. Bear in mind that to squishies or anyone who doesn't build MR, that's TRUE DAMAGE. Or sometimes even TRUER DAMAGE. That's right- YOU CAN GET THEIR MR TO BE NEGATIVE. I cannot stress enough how hilariously awesome that DPS is if you are doing amp'd damage. that's 3000+ damage to an entire enemy team, assuming you sat there with your thumb jammed halfway up your arse and didn't think to use a single q in that timeframe.

I hope you guys like Tank Karthus and that my build was able to help some of you learn him. Tank Karthus is good in the hands of a noob, and amazing in the hands of a veteran. I have no doubts that some of you will be able to get triples, quadras and even pentas in your careers with him. I wish you all the best of luck on the Fields of Justice. May the RNG be with you and the Crits ever in your favor. All Hail Lord Karthus.