As karthus, Karthus has great objective control due to the amplified damage dealt by his 1 on single targets. It will allow him to finish objectives pretty quickly once he gets some items.

Level 6 is a significant power spike as it allows him to stack up his rune, as well as finish off enemies who have managed to escape with a sliver of health.

Once Karthus gets his first item component, he will spam his abilities frequently due to the increased mana. His damage will increase as well and will allow him to gain an overall lead in the game.         As karthus, Karthus has a great objective control due to the amplified damage inflicted by his 1 on unique targets. It will allow him to finish the goals quite quickly once he has obtained some articles. Level 6 is an important peak of power because it allows him to stack his run, as well as to finish the enemies who managed to escape with a burst of health. Once Karthus gets his first element, he spams his abilities frequently because of the increased mana. His damage will also increase and will allow him to get a global lead in the game.