LoL Karthus players will find him to be a difficult character to build out properly. You will see that his build varies when he is up against either generic or specialized team comps, demonstrating you should not take the same Karthus build every game of League of Legends
The most important items to use in your champion's build are: Liandrys Leid, Rabadons Todeshaube, and Leerenstab. Those who included these pieces in their builds had a higher winrate than players who worked towards other Karthus builds. Similarly, if you are battling a well-mixed enemy team comp, you should strongly consider getting yourself the Inspiration, and Dominanz runes. In recent games, he won the largest percentage of his matches when equipped with these runes.
We calculated our Karthus build recommendations by examining 33.687 recently ranked League of Legends games with him in them. We only propose the top winrate Karthus builds that have been built by ranked LoL players enough times for us to reccomend them. With so many matches in our dataset, we are quite confident in our suggested builds.