In every guide, Karthus is always "The one that is not a threat to you with this build" So...everything basically counters him? His only real move you can use without getting in range to die is his Q and R. I know his other moves are supposed to be easy to dodge and stuff, but does it really cost that much to have a passive like his and an ult that targets people automatically? What exactly makes him strong?
zlumpy3 votesHe has amazing wave clear late game and pretty good at early game. Plus his ult makes a lot of risky plays by enemy that much riskier.
Kinda like how soraka makes trying to take someone out harder with her ult. Global pressure will always be strong unless it gets nerfed so hard that it cant do much of anything.
Dreadlocks2 votesHe has a lot of AoE damage, and can still do damage after he's dead.
In other words... 
ToNz Of DaMaGe
Eedat1 votesKarthus is dated. His main strength is his global presence with his ult. Other than that he can be a bit annoying in lane until you get boots. Thats about it
Friendly Ram1 votesthen he gets rylais.
specter4241 votesHe isn't that strong since Riot took every opportunity they could to nerf him. He can catch people off guard with a wall and q spam until u die but other then that..
Targon1 votesAccording to http://www.elophant.com/league-of-legends/champions/damage-dealt, Karthus deals the most damage of any mage and the third most damage of any champion in each game he is in on average (not sure how accurate these sites are though). But yeah, he seems to struggle against a ton of other mid lane champions.
Astôlfo1 votesLow playrate = small sample size
Reikken1 votesit's true though. His damage output is enormous, and his potential damage is demonstrably higher than any other mage
The main problem with averaging postgame "damage dealt" stats is that damage dealt is primarily affected by game length, so champions that do enormous amounts of damage but end games early rank lower on the list than champs that drag games out.
Astôlfo1 votesKarthus isn't strong.
Handy Sandy 1 votesKarthus isn't strong.
All champions are balanced, none are too strong or too weak. If they were, riot would fix it. There just aren't any good karthus players around.
Astôlfo1 votesUh yeah because Riot totally has a good history with doing that. But yeah, almost nobody plays him in the first place.
Arakadia1 votesNope. Not all champions are equal, it's nearly impossible for a perfectly balanced MMOBA.
Handy Sandy 1 votesNope. Not all champions are equal, it's nearly impossible for a perfectly balanced MMOBA.
They aren't completely balanced, but the majority of them are. There definitely aren't giant gaps in champion strength though. Many people have just not seen a good player for a champion that is considered weak or underplayed, like 
etc. Most champions are incredibly easy to use and take little to no practice to get a decent amount of kills. The harder ones may seem like they are too weak because they are way harder to use () and harder to run away or actually kill/do damage with, that's something a lot of people just can't see.
Sire Hippington1 votesHIs ult and passive are redicoulus and the rest of his kit is still rather strong. Yes, his Q can be dodged, but there is barely any spell with more dps than that Q. His E needs him to get close, but it covers a big area for substantial aoe dps. His W might not look like much, but the range is pretty high, it reveals brushes, shreds %mr and the slow gets really high at later ranks, combined with other champs it can easily creat picks, that 1000 range instant slow followed up by something like an elise cocoon or morganasnare quickly nets a kill. But mostly, there is his ult that means all lanes have to play as if their opponents had an aditional ignite, so you can't go for any close trades. Ganks also become more dangerous because there is always that extra damage from karth to seal the deal. And in teamfights, unless you get a clean fast pick on him and directly disengage, his passive+e+ult mean he will just deal really much aoe damage no matter what.
Karth is basically one of the most broken monsters you could imagin, the only thing that holds him at bay is how vulnerable his early is, very squishy, little cc and no mobillity, he's just fee food for many matchups early on, though a good karth can still farm from save range with Q and impact the map with his ult till he becomes that teamfight monster. Else there is the option to just outplay him on a macrolevel as he's very slow so you can out rotate him, but a smart tream can cover up that weakness with other picks.
Sohleks1 votesAt 3 items he gets pretty absurd damage and he gets even nuttier from there.
I play him top lane sometimes when I see a champ he can survive against (so something without enough dive * burst). Once you pick him into that situation he lamely farms top lane safely and scales into a monster, without having to worry about map presence mid lane or the range or assassins that like to be played there.
You must build him properly too. 
is very essential as it gives your ghost massive control; it also enables you to land Q repeatedly. It is essential. Because you need Rylias you should just skip RoA imo because your build is 
for mana and . From there you can get traditional ap items or if you feeling frisky 
for tank meltation and to force squishies into MR buys or recieve true damage from Karthus.
Playing him top lane is best IMO, while you need good positioning for most matchups and mana conservation particularly the first few levels he has kill pressure landing enough Q on melee and once you have rylias it's mega disgusting. I've consistently beaten amany pantheons and Fioras and such. Their gap closer isn't quite good enough. I would stay away from the Irelias and Wukongs though.
It's fun to hear that the enemy team flames their top laner while you exert map nukes and hyper scale.
Most people just aren't very good at Karthus because if you think about it he is like an immobile adc that has to skillshot every auto. However Rylias breaks Karthus. (no, just pressing R doesn't make you a good Karthus)
Oftentimes when Karthus top is on a team the whole team has such ridiculous damage his mid lane and adc can just one shot everyone. Karthus sorta performs the tank role by being a ridiculous threat that can't be stopped by killing and constantly emits slows.
CerealBoxOfDoom1 votesteamfights make karthus strong W is his strongest ability R is his weakest ability
his passive cripples some champs and enhances others
Arakadia1 votesHow does his passive enhance some champions?
CerealBoxOfDoom1 votesif you kill karthus it gives some champs something to play around where normally they'd just be doing whatever.
For example, you see Karthus and amumu coming at you side by side... which one do you focus or do you just try to bypass them to kill the others who are too far back to be dealt with without getting rolled along the way.
Quepha1 votesIn every guide, Karthus is always "The one that is not a threat to you with this build"
What guides are those?
What makes Karthus strong is massive damage output, consistently and comfortably holding both first and second place (mid and top lane) for "damage dealt" every single patch out of any champion in any role. He sits around in a teamfight constantly throwing out skittles and even if he only lands 1/3 of them that's still one skittle's worth of damage every 3 seconds which is more than most champions can say about their damage output. If he ends up a little too close in a fight his aura adds more damage on top of that, his wall clogs up fights to make it easier to get more of his damage down, and his ult is guaranteed damage against most champions and forces important cooldowns on those it isn't guaranteed (unless you're smart enough to wait for them to spend those cooldowns so you get your damage anyways). Even if he is killed he still gets his damage output and wave clear for 7 more seconds (which is longer than a lot of teamfights last).
What keeps him under control (although he's still overall pretty strong) is that all his damage is very indiscriminate. His aura just hits whoever is close to him which isn't an ideal position unless he just suicided into a baron pit, his skittles force the enemy to move around so generally only hit slower, tankier enemies with big hitboxes who can't afford to compromise their position or very confident carries who can afford to take a little damage and lifesteal it back, and his ult just hits everyone for the same amount of damage, if it hits a split pusher on the other side of the map who has lifesteal, they'll just heal the damage up on the next wave. It doesn't matter if he deals 8,000 damage in a fight before his passive wears off if it's spread around on every enemy and none of them actually die, because then it's still a 4v5 against his team and they can rotate targets and manage their resources to win. It takes teamwork and strategy to drag fights out in such a way that Karthus's damage can actually be capitalized on.
420 grams1 votesHe's good because he doesn't need a lot to scale, and if he plays passively, the ones who do need kills (yasuo, zed, Leblanc, Diana, etc) don't get kills and fall off late game where karthus does well with a few items. A level 3 ult with no items does 2750 damage to their team in total and can't be dodged.