If you think my main build on karthus is just too squishy try starting with Catalist the protector and Building RoA, don't go for WoTA and choose if you do Archangell staff, after that build him regularly.

Of course you will build dependante the other team composition so here are some ways you can do it:
ºCounter ad team composition:
- Ninja tabi
- Frozzen hearth
- Zhonias Hourglass
ºCounter caster team composition:
- Mercury treads
- Banshees veil
- Abissal Scepter

Sure you can build some of this items against a normal team if you think you are just to squishy.

If you think my main build on karthus is just too squishy try starting with Catalist the protector and Building RoA, don't go for WoTA and choose if you do Archangell staff, after that build him regularly.

Of course you will build dependante the other team composition so here are some ways you can do it:
ºCounter ad team composition:
- Ninja tabi
- Frozzen hearth
- Zhonias Hourglass
ºCounter caster team composition:
- Mercury treads
- Banshees veil
- Abissal Scepter

Sure you can build some of this items against a normal team if you think you are just to squishy.

I go 21\0\9 on masteries:
21 on ofense to maximize karthus damage, for that moment when the enimie survive you can press your R and you are sure that he will die; of course that helps to have tons of more damage in a fight on in a lane.
9 on utility for the mana regen, larger mana poll and the extra buff time.

9 [Greater Mark of the insight] ( Magic penetration marks ) are simply to make your spells do more damage. Spell penetration is a typical choice in marks, when you play a caster.
9 Greater Seal of replenishment ( mana regeneration seals ) in karthus you want to spam your Q all the time all the game but the most part of your mana problems are in the early - mid game.
9 Greater Glyph of force ( ability power per lvl glyphs ) With these your ability power late game will be really better.
9 Greater Quintenssence of potency ( ability power quintenssence ) These will give you some extra damage early game, which will grant you more minion kills and a better harrassing capability

Pros:
- [Karthus] gives huge amount of damage;
- AoE skills, all the karthus skills have AoE;
- Karthus can atack all the map;
- Karthus got a skill to regenerate his mana when farming;
- Karthus is awsome harrassing and farming;
- Karthus can win the teamfight just placing well the barrier;
- Karthus barrier have the size of a airplane!
- If karthus die he still killing all the people.
Cons:
- Karthus is squishy;
- Hard to defend yourself as a karthus;
- Hard to use the super skilled skill shot that can kill everyone if well used, you need to press your R .

Karthus