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 for Sion as a mage. However, he fulfills a unique role, in that he's somewhat of a mage-tank.
Flash is necessary to complete what's a devastating lane opener, especially with a decent lane partner. First, apply your shield, and wait until there are about 3 secs left. Flash out of bushes, apply stun, shield explodes, Ignite, whack, whack + whatever your lane partner can do and it's usually a kill.
Certainly not complete, but I'm confident that its a good build so far.