So you may be thinking, "AD jungle Karthus is the dumbest thing I've ever heard of", but I bet you haven't tried it. ;) Follow this guide and you will be pwning all the naysayers even in ranked matches. Keep in mind that jungle Karthus isn't for noobs, so unless you're an experienced jungler, you might want to stay away. I won't water this guide down explaining what common terms mean. If you're reading this guide, then that knowledge is assumed. I use this build almost every time I jungle and my average KDR is something like 22/3. I know it sounds stupid, but you have to try it! Please don't downvote until you test this puppy out in a ranked game!

Okay this build obviously has its weaknesses, but as I will show you with this bullet list, the pros far outweigh the cons.

Pros

Sexy and unexpected
high attack damage
lifesteal works great on Karthus
Sexy
Way more OP than anyone would initially think
Works wonders in ranked games where everyone sticks to the same ole formulas
Weird enough so that everyone won't steal the idea

Cons

Karthus is usually built AP. Not really a con, but some people might thinks so O.o
Nothing else. you just have to get past the stigma that Karthus is supposed to be an AP mid

With runes, you want to go all out offense. You want attack speed, attack damage, and critical chance. With these runes, your Karthus will be dealing shatloads of damage with just his basic attacks! This is great for early game because no one will expect it.

Here, you want to go all offense. Karthus isn't innately offensive, so he needs all the boosts he can get. 30 points in offense = winning. Trust me.

I'm not going to go too in depth here. Just look at the items I already posted in his main build and build him like any other AD carry, adjusting to the match of course. If you need armor pen, get it. If you feel like you want more attack damage, then go for it. This item sequence is just a good all purpose build. You definitely want to get some lifesteal pretty early on though. You need that survivability in the jungle.

Start with your lay waste and then get defile, as these will be your best creep killing spells. Defile uses quite a bit of mana, so don't worry about it until you get the blue buff. Your main attack damage will be from auto attacks, however.

Just go with smite and exhaust. Smite for jungling and exhaust for ganking. If you want something else, then go ahead and try it, but these two are probably going to be your jungle Karthus mainstays.

Okay, here is where this guide gets unique. Most of the time, karthus is an AP mid or something like that, so a jungling Karthus will throw everyone off. you should get your lifesteal item from the go and then head to blue. You need to get someone to leash you because Karthus isn't that powerful to start. After blue, do the standard rotation of wraiths, wolves, red, etc. Around level 6 you can start ganking, and this is where jungle Karthus really shines. Of course, you can chase the enemies and exhaust them like any standard jungler, but ganking with Karthus is easier than that. ALL YOU HAVE TO DO IS PRESS R. Yep, just press R to gank and you will be jungling like a pro. Everyone will be impressed that you can assassinate so well, and then you can run in and hit them with your obscene amount of attack damage after you press R. They'll never see it coming and you'll get pentakills more often than not.

Whenever you clean out the jungle or you're holding down a lane for some other fool who had to go back, you may want to use your skills to farm minion kills. You can use lay waste and defile to do damage to minions, but your best way to kill minions will obviously be your auto attack. You will be so beefy with those runes and items that are listed above, that the minions won't stand a chance. you'll be getting critical hits more than half of the time, and it'll just be GG for those minions. That's 18 or so gold in your pocket ALL DAY LONG.

When you decide it's time for a gank, be sure to alert your teammates before you do. At level 18, your ultimate won't actually do very much damage since this build doesn't focus AP, but the combination of exhaust and your fearsome auto attack will make your opponents want to uninstall the game. I once soloed a team of yordles with nothing but my auto attack and 55 health pots. Yeah, Karthus is that good.

Whenever you're doing ranked matches, especially blind pick solo queues, you just want to go ahead and autolock this guy as quickly as possible. Your team may not think that it's a good idea, but once you lock in, there's nothing they can do. You'll probably have the highest ELO of anyone on the team if you use this build, so there will be no questioning your authority. The champions that you may want to ban include Orianna, Karma, and Rumble. These guys give Jungle Karthus hell and you want to avoid them at all costs. If you do this, you should be in good shape to dominate even the most skilled players in the most competitive matches/tourneys.

So there you have it. Your guide to jungling Karthus. No one will expect it, and they may even make fun of you at first, but when you start curb stomping them with your lifesteal and attack damage, not to mention your unique gank, they will bow down and kiss your toes. Jungle Karthus is fun to play, and you'll probably never want to switch because of how hard you'll be owning your enemies. I just started using this build, and I'm already on the fast track to the top of the ladder. Good luck, and may Karthus be with you!