Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Why Mejai's Soulstealers?
Hmm at 6 lvl you have ulti and you should have a lot of frags = so many stacks = bonus ability power.
Why Sorcerer's Shoes?
Magic penetration is very important for this champion.
Why Archangel's Staff?
Archengel's Staff give a mana and ability power. You may have a truble with mana becouse Defile's needs a lot of mana.
Why Rod of Ages?
Karthus have little hp and he has to fast die. Rod of Ages give Ability Power.
Why Rabadon's Deathcap?
This hat give a lot of Ability power.
Why Abbysal Scepter?
Abbysal give Ability power and magic resist.