Karthus is a mid laner that is high risk and high reward. His well known ultimate can change team fights and a game itself as soon as it hits late game and you have that heavy damage.
Early game in some match ups Karthus can trade fairly well, however right now karthus is easily beat by a lot of mid laners so instead what you should do is use your [Lay of waste] to farm minion creeps as best as possible and poke the enemy for every creep they take with it as well. IF you are ganked use [Wall of pain] to get out. keep [Defile] toggled on if you have the opportunity to kill your enemy laner and combine it with [Lay of waste poke] as much as possible. When you hit 6 Keep heavy map awareness and use [Requiem] on anyone you see geting away with low health or if you just wanna add some damage to all lanes to help them out. REVIVE/TELEPORT: Use teleport to sustain lane and get more creeep then enemy. DO NOT USE REVIVE THIS EARLY IN THE GAME UNLESS YOU ABSOLUTELY KNOW YOU CAN GET A KILL WITH YOUR ULTIMATE. That is a very important summoner. Use it carefully.
Karthus will start geting damage here so you don't need to really keep hiding and farming from enemies clearly. What you want to do is simple: keep pushing your lane or help push on other lanes as much as possible. Keep map awareness use your ultimate when you can. Try to get dragon control as much as possible. The 1st buff adding heavy extra damage is a beautiful thing for Karthus and his ultimate. VISION WITH WALL OF PAIN: This can be used for early game as well. Using wall of pain on brushes mid game is helpful espcially with your team in the enemy jungle to catch the enemy jungler doing wolves or buffs and you can grab a kill.
Karthus is a monster at this point as he is a late game champion. Your tear is stacked you have most/if not all your items. Keep doing the same things from mid and work with your team to get those turrets as fast as possible. Keep vision on the enemy jungle to use your ultimate as much as possible to get as many kills as possible. Waves: having waves in one or all lanes is very helpful. Make sure you have wave control on all lanes so you don't get in a bad situation or a baron or possible 5th dragon is taken from you. Ward. BARON: Here is the thing about late game, Baron so karthus can die at baron and still be helpful. If your team is near you and you wanna stop the baron or atleast ace the enemy team as they are about to get it. Go in, Toggle Defile on, if you have zhonyas use zhonyas. If you don't pretty much do damage and die and press your ultimate and you should have good follow up and good damage to win. REVIVE/TELEPORT: Most critical thing here. You don't got revive and used it badly during late game it can screw you up hard. Keep watch of enemy health bars and if you get aced and they are still low use revive right away and kill them with your ultimate if you have not used it. You can use revive and just defend base or push waves in general if you don't have ultimate.
1) Go in and use defile in combination with Lay of waste and hurt the enemies as much as possible. They try to get away you use wall of pain right away and stop them. they disengage and are low? Use Requiem. You die? Use Requiem. 2)Go in and use Defile, Zhonyas to get off as much damage as possible. Lay of waste them to death. Requiem to finish them off. Karthus is unique compared to other mid laners because his job is to die at times to win the game. However that can also back fire so know when to go in and when not to go in. Communication is key. Vision is also key.
Karthus is absolutely fun to play as. He is weak early but the satisfaction of killing people across the map with your global ultimate is amazing. His damage late game is out sane and he can turn the tide for your team as he is a team fighting champion who has a 5second invisibility passive thats just OP. Sometimes it can be a humerus experience....get it? cause bones and karthus is---- Never mind, have fun. Wanna help me improve this guide? Leave suggestions and criticism I appreciate it all.