Lay Waste is very strong at farming minions and harassing enemy champions.
Ask your allies to help point out when you should use Requiem to get kills in different lanes.
The Fiddlesticks Karthus synergy stats shown here demonstrate the areas of his gameplay that do better (or worse) when paired with Karthus, compared to his overall stats. Significant differences are areas of focus that highlight how his gameplay changes when paired with this other champion.
Overall, Fiddlesticks sees a large worsening in his KDA (ratio of kills and assists to deaths) when paired with Karthus. His KDA went down from NaN to NaN. This drop shows that Karthus helps Fiddlesticks perform poorer in ranked League of Legends matches. Yet, the small difference indicates that this performance change is only marginal.
When Fiddlesticks tries to synergize with Karthus, he gets a barely noticable drop in his total damage dealt. This could signal that Karthus does not support by providing cover for him to dish out more damage or that his allyship also helps increase the round's duration. Similarly, when Fiddlesticks is played together with Karthus on the same side, he usually scores many fewer killing sprees than when he plays alone. This is contrasting evidence that he is worse when playing synergisticly with Karthus.
Moreover, when Fiddlesticks plays with Karthus on the same side, he often receives a little less damage than when he plays alone. Receiving not as much damage could indicate that Karthus does a decent job of protecting Fiddlesticks in team fights.
There is no ideal ally for Fiddlesticks that will always be his best synergy. However, Karthus is a terrible companion for Fiddlesticks. By locking into the same side as Fiddlesticks, he decreases their win rate by -7.7% to 41.3%. This proves that there is a very bad bond between Fiddlesticks and Karthus that results in them performing not as well together than without each other.
Karthus has a lower difficulty than Fiddlesticks. That means you don't need to be careful when enlisting Karthus teammates to go into battle on the same side as, as they will need a relatively low level of skill to play well with you.
Fighting on the same side, Fiddlesticks players will likely witness an overall large increase in your team’s ability to deal magical damage. When paired, these champs are able to dish out a surprising amount of magic type damage.
In contrast, by grouping Fiddlesticks and Karthus together, you probably will not be giving your side much attack damage. Plan to include other champs on your team that nullifies this relative hole in this pairing's kit.
Regardless of the ability of your Karthus teammate, you need to focus on maximizing your gold income, outlasting your enemies, and taking down objectives. If you do this, you will do well, regardless.
If you would like to truly master Fiddlesticks in order to pair well with Karthus during both the lane and mid / late game phases of League games, you should continue reading to learn a few extra tips and tricks for this champion pairing. If you listen to the build and stats displayed above, you should increase your win rate by a lot.
The ideal finished items to focus on in your Fiddlesticks and Karthus synergy build consist of Hextech Rocketbelt, Zhonya's Hourglass, and Stormsurge. When Fiddlesticks included at least these three finished items in his build, he did much better when fighting alongside Karthus than with most other common item sets. In fact, Fiddlesticks had an average winrate of 41.3% when playing alongside Karthus with these items in his kit.
To have the best chance of coming out on top against your oponents, you should equip the First Strike, Triple Tonic, Future's Market, Cosmic Insight, Cheap Shot, and Ultimate Hunter runes from the Inspiration and Domination rune sets. Out of all the rune combinations players used for Fiddlesticks and Karthus pairings, this combo of runes yielded the top win rate. Notably, these runes averaged a 41.3% winrate overall.
By default, tips, stats, and builds on how to synergize Karthus with Fiddlesticks are presented for all skill levels combined. If you would like to scope the statistics and builds to a specific skill level, you may use the selection menu earlier in the counter matchup guide.