Karthus’ Ultimate R is an excellent tool, and it can allow him to impact the map from afar.
His afterlife can single-handedly change the tide of a fight, especially if the enemy continues to fight near him after he dies.
Karthu’s early game is relatively weak. This can make it hard for him to survive the laning phase, especially when he is up against a solid early game champion.
Karthus is a farm heavy champion and will need to soak up many minion waves and jungle camps to be useful throughout the game.
His Ultimate R deals basically no damage in the early game. This isn’t good for Karthus as he only wishes to use it when the enemy is super low. During this time, the enemy could have already been killed by your ally, and you wouldn’t get an assist.
Channelling your Ultimate R near an enemy champion could be very dangerous as they could interrupt the channel with CC.
0 - 15 min
Try to go for short burst trades with your Q in the lane. The goal is to ensure that the enemy can't all-in you during the laning phase. Ward around and just aim to play safely.
Once you get your first item component, you should spam your abilities more frequently in the lane. It will help you use your W combined with your Q to go for regular trades with the enemy. You still need to play safely, though.
On getting your Ultimate R, you should be able to finish off kills in other lanes. Make sure that you show up to neutral, objective fights as well since your E will be really effective in those situations.
15 - 25 min
Getting your core items will let you run down enemies and burn them to oblivion. Use your Ultimate R at regular intervals to ensure that you can help your teammates.
Fight near chokepoints to maximize your team fighting effectiveness. Sometimes, it would be wise to throw yourself into the enemy team, especially if the enemy frontline decides to all-in without any follow-up.
Another point in your Ultimate R will let you destroy the opposition due to your consistent burn damage. Use your E properly to make sure that you win clumped fights.
You will be able to blow up squishies with your Ultimate R with ease now. Even if they don't die, they will be too chunked to participate in a team fight.
You will be able to burn through the entire enemy team with proper positioning. Your Q combined with your W will be able to one-shot enemy squishies. Use this strategy during neutral, objective fights.
Remember that your W placement can make or break an entire team fight. Place it well and from the fog of war so that the enemy team doesn't anticipate it and take necessary precautions beforehand.
His early game is fragile, and he can easily be caught out if he is not careful. He should focus on farming and scaling up with occasional burst trades.
Level six is a massive power spike. He can quickly look to use his Ultimate R to secure kills throughout the game's early stages. This will help him scale up quickly.
The first item component will cause a massive powerspike for Karthus. He will not run out of mana quickly, and his damage will increase significantly.
Adding 2 points into his Ultimate R will cause another significant power spike. His ability to burst down low-health enemies will dramatically increase, causing mayhem in the enemy team.
His Q will be maxed out at level 9, which means that he will deal a lot of damage to single targets. It also has AoE properties which will help in clumped fights.
Karthus is really good in team fights as he can slow down multiple enemies and deal a lot of damage to a group of people. This will keep on increasing with his item count.
Karthus is really good during the late game. His abilities will hit like a truck due to his items. His Ultimate R will be pivotal to winning the game.
Once he gets 3 points in his Ultimate R, it will allow him to half-health squishy enemies easily. This means that anyone who is behind on the enemy team will suffer a lot.
At this stage of the game, Karthus would've maxed his Q and his E. His damage output in team fights will be very high.