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 has two abilities that scale tremendously with AP (1 to 1 AP scaling), in my opinion he is one of the strongest characters when fed often two hitting champions. This guide attempts to use his powerful AP Stun and Shield to their full advantage.
Magic pen is amazing with any AP based character, the same applies for Sion. We take HP seals because it's the only thing worthwhile getting, helps you lane a bit longer. Cooldown glyphs so you can spam stun/shield more often.
Cryptic Gaze provides a flat 2 second stun for all its levels, and also features a 1 to 1 AP scale. Not much to it though, just click, cast, stun and damage.
Death's Caress is an amazing ability, it costs almost no mana, does large AOE damage with 1 to 1 AP scaling, and provides a temporary shield. It does however have a 4 second cooldown before you can manually pop it to deal damage and if your enemy damages it to 0 it will deal no damage. That is bad. Always try to engage enemies with a shield that can be manually popped straight away. You want the damage more than you want the shield.
Enrage provides your AP caster with a large supplement of health and attack damage, something that almost all other AP casters lack. You want 1 point into this fast, it will add up.
Sion is the only character where you don't want your ultimate straight away. Totally based upon dealing physical damage it's not much use to our AP caster. However, it does heal a significant amount, especially when paired with Lich Bane, once you get used to actually casting it, it will save your life sometimes and is great late game.
You need the extra mana, Sion doesn't build mana very well naturally. Only use when you reaaaaally need to, it will get you extra kills if used right.
You will be travelling the map often, trying to feed off the side lanes. You need teleport to get back into your lane when needed.
This build largely relies on Mejai's, Sion is all about the stacks. Try get kills as early as possible, if you miss your chance to feed you will become useless, if not you WILL win. Taking mid with this build is advisable, and if you cant feed off your opposing mid, go to the side lanes, its almost always worth the trip.