1st Guide, so don't hate. AP Sion does gratuitous amounts of damage and is rather beefy after farming with Enrage. The play style I suggest is rather aggressive and requires a competent laning partner. I hope you enjoy my guide and playing AP Sion before they nerf flash through the floor...
Constructive criticism is always welcome and feel free to rate, thanks!
Mpen Marks and Glyphs: Early game Mpen will make you terrifying in lane and provide you with decent farm from your shield
Flat AP Seals and Quints: Your AP scaling moves each have a 1 to 1 ratio so early game AP is essential. Since you're not going to get any AP from items until around 12:00 minutes (ROA) you need some amount of starting AP. With the master point in Ignite you can begin the game with around 32 AP, which, when combined with your Mpen, gives you decent starting damage.
Do not purchase scaling AP runes of any kind. Sion is one of the best farmers in the game. The combination of Cryptic Gaze and DC lets you take an entire minion wave for 200 mana.
Rather self-explanatory. Cryptic gaze allows you to stun and do good damage to a single target but it won't be your main source of damage early game. Enrage is important to have a rank in early game, it gives you a good amount of bonus AD and you can farm health throughout the game using it. However, your real damage output comes from power-leveling DC. Stun to shield nuke should half health most non-tank champions that you will encounter in lane.
ROA. Get it as soon as possible so it can fully cook as soon as possible. After ROA, shoes, and Deathcap, all items may be considered situational. You may wish to acquire some resistances or you may wish to get some CD reduction. Either way, the first three items are crucial to your damage output and survivability.
Do not purchase Lich Bane unless you are actively carrying your team. Lich Bane is REALLY ****ING EXPENSIVE, and, however good you are at farming, it still remains the same exorbitant price. Also, remember that Lich Bane requires decent amounts of AP before it will really give you that extra nuke. If they have champions that you do not want to enter melee combat with (Xin Zhao or other tanky DPS's) then do not purchase Lich Bane at all. Sion also lacks the ability spamming needed to properly trigger Lich Bane. Don't buy it.
Some may question Zhonya's Hourglass. I don't buy it because I think that if I enter a team fight I will be focused and die, I buy it because if the enemy team can break my shield before I am properly in their midst, then half my damage and all of my AOE are gone until it cools down. Zhonya's lets me activate the shield, flash in and wait for it to allow me to detinate, and then do massive amounts of damage to their whole team. Zhonya's > Lich Bane.
Flash and Ignite. Unless you have a Zil. Zil will facilitate your team fight nuke with his movement buff. Otherwise, Flash and Ignite. Ignite gives you more early game AP. You want early game AP. Flash is useful for escapes, 1 on 1 combat, teamfights, and stealing baron. After they nerf flash, ghost or exhaust may be my next staple but ,until the patch, Flash will give you good mobility in teamfights and great escapes in lane. When is Flash not useful?
There are two ways to play AP Sion extremely effectively.
1. Aggro carry: Sion can put a team on his back and walk around with it. Unless your allies have fed the enemy team's damage output champions, you should be able to nuke their team and take turrets like a boss. This will work extremely well if you have a Zil. Zil makes AP Sion own face. I like Zil.
2. Supporting a carry: I tend to lane with my friend who plays Jax. I'll use my stun and shield burst to take out half of one of the enemy champion's health, then Jax (or any other damage output champion) hops in and destroys them. I can feed my laning partner FB by 4 minutes and future kills a plenty if he lets me farm while he stands there and looks scary.
In the image of my match history for the past few games you will see that I only opted to carry once, preferring to aid my Jax in most of the games (bunch of assists). Supporting a carry while farming like mad is the most useful way to play AP Sion for your team.
IMPORTANT: If you do this, DO NOT TAKE KILLS FROM YOUR CARRY (early game). Do not split feed up between the two of you. Either take ALL of the kills, or take none of them (but also never let the enemy escape). Do not jockey with your laning partner for kills if you aren't going to carry the game.
Power level DC, it is your damage output and it gives you amazing laning potential (pull minion and champ Aggro and just forget about it). Cryptic gaze and DC do a lot of damage. Your Aggro begins at level 3 when you take your first rank in DC (Enrage comes before DC due to the damage buff and farming passive). As AP Sion your ulti is not that great =/ It's single greatest use is jungling or healing yourself and an ally in lane (But don't hesitate to ulti in teamfights).
Ahhh, the best section of an AP Sion guide. Sion can farm like a boss. Once you have ROA, keep Enrage turned on, Cryptic gaze the cannon minion, hit each sword minion once, and burst DC. Minion wave gone. As you build more and more AP you will only have to Cryptic Gaze the cannon minion and burst your shield to farm the whole wave.
When to farm? ALWAYS. Never stop farming.
Walking from bot lane to top? Queue up DC as you pass dragon and burst it while walking through the lane to get the minion wave.
Running from the enemy through their jungle? Queue up DC and burst it as you pass a minion camp to get some more gold.