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 is all about building utility through out the initial levels of the game. The catalyst is for survivability in the early stages of the game, i dont usually upgrade this untill i hit lvl 17 or 18 when the proc on it is useless. I keep the tear for as long as i can to increase my mana and make 'Q' ability OP. I grab Mejai's before i upgrade the utility items as i see this the perfect time to get it. If you not raping hard and getting kills before you get mejia's, i would suggest skipping this and getting a flat AP item (i.e. ryalis.) Once you have mejias, it timeto start building Archangels and RoA. Once u have these 2 built, i then grab Zhonya's and Void Staff, however, your last item is really situational.
This build i find works well on other AP chars like fiddle just aswell.