Revive and teleport is not best choice on Karthus. Also you can show masteries at OFFENCE. All other is normal.

You have Rod of the Ages, Archangels Staff, and the mana seals. Karthus doesn't need that much mana because of the passive on Defile. I'm not saying he doesn't need a little, but you are overdoing it. Also, I have to agree with everyone else that Revive is not great because of the giant cooldown on it.

In regards to the post above me, do not run Clarity

In any event, Don't take Revive and Rez quints totally pointless, also throw the death reduction in utility out, there's no reason to run all 3 of those
- you should definitely be 21/x/x, Executioner is too good to pass up especially with Karthus' ult

additionally, Guardian angel is bad, randuin's is situational not core, don't start doran's ring, no deathcap wtf?, archangels isn't really necessary

If you're going for a tanky karthus consider the following options: Zhonya's Hourglass, Abyssal Mask, Rylai's Crystal Scepter, etc.

Take it from a guy who has mained Karthus since lvl 15 (year and a half ago) and can and will take him any lane or jungle.

Archangel's Staff is not needed really, Rabadon's Deathcap is needed. ROA is the way to go for first item. While i like what your trying do with a tanky Karthus your going about it the wrong way. get rid of Randuin's Omen and Guardian Angel and buy Rylai's Crystal Scepter and then either Void Staff Zohna's Hourglass or Abysall Scepter depending on situation. (also if you want to be annoyingly tanky you can put a Warmog's Armor on him and make it work).

Lastly get rid of the Revive TO be honest I run Heal Clarity or Flash Clarity (junling is difrent but that has no point here so i will leave those options out).

Lastly runes should be mag pen makrs, armor or mana regen seals (either one works), flat ap glyphs and quints or flat ap glyphs and spell vamp quints.

also you could really use a chapter on your items (talk about the core, possible situational items, and stuff like that.

Very good start though just needs some tweeking in difrent parts.

I would not use revie on this toon. There is a reason q dodge when they see your packing this: it stinks. First off the CD on it is horible, and your straight revolves around you dieing. Generally your goal is to not die unless i have this game wrong. I would recomend takeing flash and telaport. However if you really want to do a full death defied then revie w/e. You still need to offer alt runes and deathcap is esential. Good luck and keep it at bro, nice start.

Uhh don't listen to this please. You need Rod of Ages on Karthus and you need it as your first major item. You can still buy Tear of the Goddess first and start working towards the mana stacks without buying the entire archangel's staff]. Without RoA you are way too squishy and will die immediately, and dying on Karthus is not the end of the world but you don't want to be dying immediately, because then you can;t move when enemies walk out of your [[defile... Also you have to have Rabadon's Deathcap I don't care what you say this is the one item all mages should be building. Next issue: If you're going to take Revive do not take Greater Quintessence of Revival because that's just counter-intuitive and plus, you're writing this guide for people to actually replicate, no one is going to buy those worthless quints. Pick up 3x Greater Quintessence of Ability Power or something useful. Also, Randuin's Omen is uselss on you, save it for your tank, and Guardian Angel goes against everything your build is trying to accomplish. You have a summoner, runes, passive, and now an item that all serve the same purpose, cut them out your passive is enough of an advantage.

the build is ok but take those tips

First: Revive doesn't worth since it has a too long cooldown, so think about getting something like flash to get away when u need or clarity to stay forever in the lane

Second : I should recomend u start with archangel instead of RoA. The reasons are : It stacks really fast as Karthus, solve ur mana problem, give u more Ap, RoA makes u look like too beefy and as Karthus is good to die in the best place possible and soon, so they will rather focusing ur carry (that doesnt have an afterlife) instead of you and its bad.

BTW the guide is cool, keep working on it