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 inititate 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. (This is not advised as it was before due to shorter time of remaining "alive" after dying)
 * 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, toggling on, and finally endlessly casting . This may actually get you a kill.
 * can't use summoner spells such as,  ,  ,  , or  while in his  state.
 * 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 is near manaless, 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 it's 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 you're 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 MR. This means that if your enemy has 54 or less MR before the reductions, you will bring them below zero magic resistance.
 * Be careful of when to use due to your AP and the enemies magic resistance.
 * Using  will deal damage every second to your opponent after activation. However, it will deal twice as much damage if you activate it and immediately deactivate it (and so on), because it deals damage right after activation and has a cooldown of ~0,5 seconds or lower and deactivating has none. Keep in mind, this trick will also cost twice as much mana.

Build usage

 * Carrying or  can lead to some surprising tactics with  and increase your overall damage output. Note that the slow of  only works upon activation of.
 * can enable you to regain health quickly with or, especially with enemy minions around.
 * 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.

Countering

 * Playing against can be extremely frustrating, as avoiding death with a small amount of health left is nearly impossible with 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 AD 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 the  is.
 * Always remember, especially during team fights, that remains able to cast spells while he's dead, with the bonus of no mana costs. 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 weary of this if you are playing a squishy character and/or have little MR, as damage output is steady but deadly.
 * Also, do not underestimate the tick on his . Its damage is over time, so it may not be easy to notice, 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 will 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 MR, it doesn't just penetrate it. This means two things: Damage done to you by all enemies will be increased, and as a Reduction, it has the potential to bring your resistances below zero, meaning damage against you will be increased by a percentage. 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.