My goal is to DEAL MASSIVE DAMAGE USING DEFILE in 5 vs 5 fights mid-late game.

I think that Karthus isn't only a "R" key on your keyboard.
Karthus can control the game, slow enemys, low enemy magic resistance by huge amount, deal massive damage with Defile, and eventually finish running enemys with his Requiem.

Karthus is easy to play if played safe, or if you kamikaze+defile+requiem, but if you want to deal MASSIVE damage late game with Defile without dieing it becomes much more harder!

I prefer Magic penetration+ 400 AP+ armour and HP; instead of a full AP karthus.
Full AP karthus is only for a strong requiem. But laywaste and

I DON'T go only AP because i dont care too much about requiem, so i will have a 3/4 effective requiem but a lay waste and a defile more effective, many HP/armour and hourglass that gives you +2seconds in combat without tanking anykind of damage.

Marks, Magic Penetration. no comment on that ;)

Seals, you can go health, mana regen, but in a 5 vs 5 i think +25 ARMOUR helps much more than 200 hp or mana reg.

Glyphs, again i wana explain that + 30 AP means = more or less +8 damage on laywaste/defile -.-; I think that +25 Magic Resistance means that your "effective healt" has a +500HP boost!

QUINTESSENCE: you can go with Magic Penetration to have +5,5% overall damage but the effect of this will become Half on late game because of enemy magic resistance and your void staff.
If you have to controll the game you should be a bit faster than most enemys; that can save your life, or condamn an escaping enemy with your wall of pain.
Plus, it helps much early game in solo mid to avoid catylin, brand, swain, morgana, lux, AoE attacks and stuns; and to avoid to got ganked.

no comment about that.
1)15% magic penetration (9% if you got void staff too)
2)Mana Reg.
3)CD reduction and Summoner spell CD reduction
4)move speed

1 vs 1 improve laywaste mainly
many vs many improve the defile

Clarity+ flash

Clairty in for mana regen early game, and for mana pool late game.

if you want use rod of aegis you can use ghost+flash, and remember to add ghost in mastery too.

Instead of Randuin's Omen..
..you can add Rod of aegis, or Warmog's Armour, or Rylai Cristal scepter.

1)early game dont waste too much mana, and play very defensive up to lvl 11.

2)on 5 vs 5 go aggressive: go do a wall, activate defile, activate hourglass, activate Randuin's Omen, activate flash, run away with 1 hp, use requiem, help your team to finish some1 or help some1 to escape with your wall. than recall or push.