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.
Sion - The Ap-OP
You can switch magic pen quint for ap quints. If you dont die in early game, when you get deathfire grasp, you'll find yourself doing a huge damage with the stun and your shield and to finish the combo you must use the deathfire grasp active. Remember to use exhaust and your ulti for effective ganking.