Alrighty, rune page wrong, you already get destroyed by most matchups in mid, at least helpyourself by not feeding 10 kills in the first 5 minutes, go mpen reds, armor yellows, mr glyphs, ap quints or ms if you fancy. Boot's aren't worth it the two choices for Karthus are Crystaline Flask or Dorans Ring. Karthus needs some sort of mana source so either build Tear and RoA or both (starting tear first). Morellonom is not worth it on Karthus early You q is already on a low enough cooldown and you need to mass ap to make your ult do any damage at all. DeathFire grasp is terrible you play karthus to kill the whole team with aoe (and suiciding in teamfights mostly) deathfire grasp does not match this playstyle.

I welcome all criticism since i want my guide to be as good as possible, so first of all thanks for the comments. Didn't notice i spelled Requiem wrong haha, it was late and english is not my primary language). I also agree that the texts are unbearable to read and I will work on everything you pointed out.

you need to put some bbcoding into that. Also, some WALLS of text, please, fix that, no one will actually read your texts the way they are. Your runes are ******ed, the thing is you dont pick seals of ap if you are vs example Zed or Talon, you need to do a section to explain why you pick these runes and why you pick them over any other runes.

huge block of text. Makes it hard to read. I also recommend that you use BBcode in there.

I would like to talk about your guide's cheat sheet:

Summoner Spells:

• While you added situational spells in the guide, you should not use ignite in your cheat sheet.

Runes:

• Greater Seal of Scaling Ability Power is bad. Replace it with Greater Seal of Armor. Also, while Greater Glyph of Ability Power is good. You should probably consider Greater Glyph of Magic Resist.

Items: (Oh boy here we go)

• Starting with Boots is very, very bad. You should also increase the amount of potions to buy early with boots. (Remember I'm talking about the cheat sheet only)
• No Tear of the Goddess rush. Don't be silly, it should be a core.
• Morellonomicon is a bad option on Karthus unless the enemy has certain champions.
• Deathfire Grasp sucks. Karthus is more AoE based, and the range of his skittles is bigger than the range of Deathfire Grasp.

Masteries:

• 1 point on Meditation > 1 point on Phasewalker .

Now last but not least, you spelt Requiem wrong in your title -.-