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 is a kind of Ap/Hp build. The point of this build is to flash and owned the enemy team with all that aoe plus the sunfire, ap and the abyssal you do a lot of damage. Anyhow the ultimate has a very high cd so with the frozen you fix a little that problem.
P.S: Always try to flash when the entire enemy team is together so your W, E, R have more effectiveness