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.
This build gives tons of AP and Health. Try to get mid it really helps, i always dominate every game with this build and mid.
I start off with Amplifying Tome and a health pot and lane till I have 2500g and get Mejai's, boots, and a Giants Belt, if im forced to go back before i have 2500 i get Mejai's and a Giants Belt. From there just follow item build and you should have 600-700 ap along with 4000-5000 hp.
As for the last item i left it off cause it always depends for me. If your getting tons of stacks with Mejai's then i will get a Guardian Angle, if not I would most likely get another Rylai's or Abyssal Scepter, and a Void Staff if a lot has Magic Resist. With runes, boots and Masteries, the Void Staff would not reduce the enemies Magic Resist by a lot unless its Stacked.
If the game lasts even longer i sell boots and buy Elixirs of Fortitude and Brilliane and rebuy my boots, which ive done in many many games.