good job

I think masteries should be 21-0-9, and I don't like your mid game item sequences. You also talk about the passive 'Death Defied' quite a bit from what I read. The whole goal is not to die regardless you're giving them money, and you definitely can't guarantee a kill after you die, it's just an impractical way to play I think.

+1, in-depth guide :)

Thanks for all your comments guys.
@sn0wwarrior, wow that was ******ed of me, only five items, it should be fixed now. As for your point about still being squishy, i find that after you have Guardian Angel and Zhonya's, by the time the protection from those two items passes, the enemy is focusing someone else, and if they do return to focusing you, they are wasting their damage, because the 7 seconds from requiem should last the rest of the team fight at that point, so you will have been dealing damage the whole fight while absorbing plenty of damage. As for the summoner spells, clarity is really only useful during the early game phase before you get archangel's staff, so i leave that out. And i think i explained my reasoning about heal in the summoner spell section.

@NicknameMy, I almost always build 9-0-21 on casters, and just for evidence of this being a good idea, the current top Karthus guide shares that layout. I probably should actually change summoner's wrath to arcane knowledge shouldn't i, i'll do that now, thanks. As for revive, i really think its use is massively overestimated. In early game, the team fights should leave them low enough for you to Requiem and kill during Death Defied, and in late game, everyone will have life steal, spell vamp, or a banshee veil that just makes the time between dying, reviving and casting too long to leave them on low health.
I know pics and videos would make the sections better, but i have not yet got around to making them.

@Demon Cleaner, Thanks for your reply.

Good job. Makes me wanna play Karthus again, but whenever i do again i forget about his weak laning^^

Where is Revive in your summoner spell section? You know Revive+ Teleport suicide Karthus?

21/2/7 is the best mastery for Karthus with all of AP from offense tree.

Hey, great looking guide so +1

However a few points that might help you.

I main Karthus and I like your concept with the instillation but your still too squishy for that so I would recommend a warmogs for the tankiness. Also you only have 5 final items so maybe Abysall scepter or rod of ages.

again good start but you need more then just the 2 sec from hourglass active.

lastly I would move heal to an acceptable summoner spell and also add clarity as an ok one.