hi I'm wixa and I am one of the best karthus players world (top 5 world and rank 1 peak NA)I mostly play Karthus and Kindred and stream often on weekends so feel free to tune in and ama I'm really informative.

https://prnt.sc/1tg49pf
https://www.twitch.tv/wixalol

Karthus is a really easy and good jungle pick always. Karthus offers a lot of damage and you as Karthus should be getting most damage every game. Karthus is a good scaling champion, even if you do bad on this champion you still do something with your e and r . Karthus has a low ban rate and I'd consider it a good pocket pick because if enemy would rather waste a ban on this then 9 times out of 10 a better pick is open.

-I like rushing sorcs; damage and move speed feels really good just make sure you're managing mana correctly early
-Going codex is better than lost chapter
-I don't like going ludens but it does have more damage liandry
-depending on enemy champs I'd go demonic over void in some games (Mostly for bruisers or champs that build hp such as riven xin)

Last stand - Last stand gives more damage than coup but coup offers snowball and it works better with your r. In general, you should be taking good fights so that being said last stand is bad.

Ultimate hunter - Ultimate hunter shaves ~30-40 seconds off your r but I would much rather take ravenous

Inspiration/sorcery secondary - Inspiration is good higher elo but it offers a lot but there's runes I like better and sorcery gives a lot of ap if you take gathering absolute focus

2 full clear rotations and a bit of minion xp + crab give level 6 (should get it around minute 6)

You can dive really easily on this champ just be mindful of where people are on the map

Karthus can space really easily unless enemy jungler can get onto you easily but either way Karthus wins most 1v1's. Champs that can invade is annoying to play against but your champ scales.

I recommend banning Kayn or Xin zhao

Diving is really good on this champ especially if you take ignite

Save smite for crab always on this champ

your q scales with cdr I would recommend not spamming it early game when you're dueling

I like sitting on stopwatch and it's even more worth to buy if you have a bounty

Catching waves on karthus gives a lot of xp and gold

even if you have a bad early game you can still win later on especially on Karthus