Karthus/Strategy

Skill usage

 * Sometimes it can be beneficial for to die a strategic death, by rushing into a group of enemies and casting  the enemy will focus you long enough for your team to initiate without affecting your chance of winning a team fight, as  can cast his spells even after dying.
 * You can also activate right before dying to let  tick even longer. You can also use  when dead to mow down the person who killed you.
 * His passive allows him to cast spells while dead. Abuse this every time you die and punish any who remain near your corpse, casting if not already used, turning on, and finally endlessly casting . This may actually get you a kill.
 * When focusing on harassment, try placing where it will hit the champion, but not the minions. This way, it will deal double damage.
 * is a great harasser. Use your to zone your enemies.
 * can be used to scout brushes since it reveals a small area around where it is cast. Since the skill has a low base cost, this can be used to scout brushes frequently.
 * is very strong at farming minions and harassing enemy champions.
 * Taking 1 level of gives you a slow allowing you more time to cast  on fleeing enemies, as well as deterring attempts of harassment, even at their towers. In later game it is useful in team fights, thanks to its armor and magic resistance debuffs. With this spell,  can actually be a very good ganker, as it slows and allows his teammates to deal more damage.
 * at higher levels can mow down creeps by standing next to them or walking past them for just 2 seconds. If given free rein or adequate protection from enemy champions, this can turn the tide of a battle very quickly or cement a teams victory.
 * Ask your allies to help point out when you should use to get kills in different lanes, or alternatively, alert your teammates when you are about to use it. However, be sure to keep your eye out for action elsewhere on the map; you may be able to help someone when they otherwise wouldn't have time to type for help.
 * Before using to kill a retreating enemy, consider the shield/heal/defensive abilities and items of the enemy team; at endgame, it is usually more important to have an ultimate up than to force enemies to heal.
 * During laning, it may be more beneficial to attack the caster minions in the back and to kill them than to harass an opponent. If you find yourself being outlaned by the enemy, pushing them back and stacking up on your own minions can help control them and keep them away from your tower.
 * If your team can effectively distract the enemy, can actually be a very effective pusher, as the sheer speed at which he can consistently clear minion waves allows him to push his team's minions to a tower and destroy it in a short amount of time.
 * highly benefits from the buff, giving him increased mana regeneration and reduced cooldowns for all his spells, allowing a focused AP build.
 * can be used mid-game before a major offensive is launched to weaken all enemies, making them easier to kill.
 * and are useful summoner spells, as  often needs mobility in order to be an asset to the team.
 * It is possible to jungle using Karthus with the proper runes and/or masteries. To get the blue golem rune, use only on the blue golem (missing the smaller monsters). This way, you will deal double damage to only the golem.
 * Having and  will allow you to ignore 75 magic resistance when using . Take this into consideration when attacking an enemy, as you may be able to do more damage than expected.
 * Also, the and  reduce magic resistance. This means that if your enemy has 54 or less magic resistance before the reductions, you will bring their magic resistance to below zero.
 * Be careful of when to use due to your ability power and the enemies magic resistance.
 * Using will deal damage every second to your opponent after activation. With no cooldown reduction, it has a 0.5 second cooldown on both ends (activating and deactivating), making it deal the same damage as just leaving it toggled on. With any cooldown reduction, however, you can increase your damage by rapidly toggling on and off. At 40% cooldown reduction, you can effectively deal damage once every 0.6 seconds, instead of once every 1 second, for 1.67 times as much damage
 * Both slow and stat reductions go through.
 * can be a viable tool to saving an ally champion's life due to its movement speed reduction. Use it to give your allies a chance to get away if they are running from a pursuing champion.

Build usage

 * Due to, the enemy team will likely build some magic resistance. Building magic penetration items such as will help pass through their defenses.
 * can enable you to regain health quickly with or, especially with enemy minions around.  also gains full spell vamp when only used to attack a single enemy.
 * You may want to consider rushing and then getting  on , as he is very mana hungry and tends to use  a lot for farming/ganking purposes.
 * When played as more tanky build, consider as a top-priority item, since it's provides slow for all your spells, but especially for, which you can use to kite enemy with terrifying effeciency.
 * Karthus has a high base mana, so using 4 s (and 1 ) he can get among the highest ability power amount in the whole game. Normally, you get a lot more ability power from stacking s than due to Karthus high mana so to get the most ability power (Assuming you use one slot for boots), consider getting 4  instead of stacking.
 * That's however not counting any masteries and runes. will give you 5% more ability power from the mana.
 * However building as a glass cannon (only buying offensive items) means that you will have no survivability in teamfights if you end up in the middle of one, although if you are ready to die for your team, you can deal a great amount of damage to the enemies post mortem as long as the teamfight is still around you (or to be more exact, whats left of you).

Countering

 * Playing against can be extremely frustrating, as avoiding death with a small amount of health left is nearly impossible due his . If you find yourself having trouble with an enemy, buying a  can offer at least some protection to his ultimate. If you are a caster and wish different/more protection, grabbing a  earlier than you normally would can save you on many occasions. If you are an AD-based champion and wish for some additional protection, a  can block 300 magic damage from his ultimate if he takes you below 30% health. Beware however, the  isn't as reliable as.
 * is a good counter to Karthus's ultimate during the early and mid game. It costs only a fraction of the other items which would normally be used to counter.
 * Always remember, especially during team fights, that can continue to cast spells while he's dead without the cost of mana. When engaging his team, kill  first, then retreat out of his attack area before continuing the fight.
 * When in the midst of a team fight, it may be difficult to notice the attack animation of his . Be wary of this if you are playing a squishy character and/or have little magic resistance, as damage output is steady and deadly.
 * Also, do not underestimate the tick on his . It is DoT, so it may not be noticeable at first, but its damage can be as much as 160 per second, not counting resistances, on top of his damage from.
 * Do not underestimate 1v1 potential. He may be squishy, but his killing power vastly increases when fighting stationary, single targets. If you engage him as ranged, try to stay out of his  and keep moving to avoid his . If you need to remain stationary, stay near other targets that ralwill be damaged by his  (e.g.: Allied minions or neutral monsters) to reduce his damage output. If you engage him as melee, you should either only do so if you have the level advantage or survivability to engage him in his AoE spells and live.
 * reduces armor and magic resistance, instead of penetrating it. This means that damage done to you by all enemies will be increased. Keep this in mind while standing in or walking through his during a team fight.
 * Some players will intentionally walk into a teamfight and die, This is rather a great strategy if  is in a situation where he will die because he can cast his spells for 7 seconds after his death allowing him to freely channel . Make sure that after you kill him, stay away from his  AoE.