Hi! I'm BenLegend443. I play on NA(plus TW and a bit of KR) and enjoy playing Karthus a lot. I've only been playing League for approximately a year, but I read up on game info a lot. I'm only Silver 4, but I've got a Mastery 7 on Karthus. I like to play him for his satisfying ultimate. Sorry if the guide sucks. This is my first time ever making a guide, and please don't flame me for not being a great player(or guidemaker)!

Pros-
-An absurd amount of damage
-Lots of probable rune paths
-Easy to pick up
-Satisfying
Cons-
-Enough counters that it isn't even funny
-Immobile
-Squishy, although may be favorable in some situations
-Needs items to be strong
-Mana hungry

Karthus is one of the biggest damage dealers in the game. He has been since the earliest days of League. For runes you can take whatever you want and probably get away with it(for example, Precision primary won't work, but Precision secondary will)The optimal build is to just build a ton of damage. Since you're not a backline mage like Lux or Xerath, there is a playstyle that is called suicide Karthus. You're especially suited to this because you've got your passive, for some extra destruction. Karthus is a battle mage, one that goes to the frontlines and kills everybody. Hell, you can even flash into the middle of the enemy team. Poke with a few Qs, turn on , immediately in, then throw the W wherever the hell you want in the enemy team since it's probably chaos at this point. Unless they're incredibly coordinated, they're going to die. A good side effect to this is that some of their important cooldowns are used to kill you. They won't have much to deal with the rest of your team afterwards, and you'll be in passive, free to wreak havoc. Dark Harvest Karthus is good for this since it will constantly proc and give a lot of extra damage.

Another playstyle is to poke and play with your team, then ult after the fight ends. This is called tactical nuke. Place your somewhere where the enemy will walk through, and constantly throw s at them. The point of this is to poke them low enough and bait them into using all their heals,abilities like Master Yi's Alpha Strike and Fiora's Riposte, and . At this point, feel free to ult. Remember to charge (if you have it), , , and up to full stacks before ulting, or you will not get the full damage. Any champ that has a destructive ability like Hecarim with his is a good synergy as they can bait out counters like Kindred R or Soraka R.

As mid laner, it is common practice to roam, but as Karthus, it would be a bad idea(cuz no mobility), and why? You've got a global ultimate! Your ult can chunk the enemy carries for about 4/5th of their health, and their bruisers for about 1/3. Farm and poke up until mid/late. Just stay in your lane and farm, while clicking R whenever a good opportunity presents itself. Win lane, win game. Now get out there and dominate!