This build gives you maximun of magic penetration

This build gives you health to resist

This build gives you spellvamp and life steal to resist

I made this build after 1 year of playing Katarina, now she is my main.
I tried lots of builds with: Rabadon, Zhonya, Deathfire and all this stuff and I went to conclusion that this is the best build. It gives you everything you need.
I did lots of pentas with it. Of course, it's about the way I play it too.

Start with Q, then W and after this E

Max Q, then W and after this E

Max the R asap

# Why max Q first?
Q puts on a mark that is consumed by an auto attack or spell

# Why max second W?
Because it has lots of damage and it's best in combination with Q

# Why max E last?
Because it's used to jump and does not deal that much damage as Q and W

*SPELL ORDER*
Start with Q then E then W then R
-OR-
Start with R then Q then W then R

Use Q and be carefull with the mark

Use Q in combination with W

At lvl 1: with Q and auto attack (be careful!)
At lvl 2: Q and W
At lvl 3: Q - E - W OR E - Q - W