First off, welcome to my Sion Guide, I hope you find it helpful and fun.

Sion is a very cool champion and I have loved and played him through his nerfs and his buffs and as of late he has been coming out as a strong force in ranked gameplay. I have seen people play him many different ways AD and AP and this build is just what I find is most effective on him and will get you the most wins, facerolls, and enjoyment. So enjoy :P

Pros:
+High Damage
+Loads of Utility
+High Sustain and Tanky
+With this build very fast and mobile
+Has a large Hammer

Cons:
-A tad bit of mana problems early game
-Death prone
-If you get focused your sheild can break so can be hard to posistion right and determine when to enter a team fight or brawl.

AP Sion is an insanely high damage burst mage/farmer/pusher.
Sion has two damaging ability one toggle ability and one activation ultimate.
You want to have your toggle ability on at all times the minor damage you take for using it is easily mitigated with Sions high natural health regen.

When Laning with AP Sion you want to take a solo lane preferably middle against a mage who has problems with you getting in his face and zoning the **** out of him.

When in lane you want to stay ussualy between the caster minions and melee minions with your sheild on which will push the enemy out of farm range so he can stay out of your range while you being in a perfect spot to farm minions, whenever your enemy gets close to farm punish him with a Q to W burst damage, every time you encounter a new minion wave you want to start up your sheild and if your enemy stays away from you too long for you to use it on them just use it to last his a couple of minions. When you are playing Sion you should be faceroll farming and constantly making your opponent go recall and if they dont you flash in there and kill that guy.

Sion will want blue buff it makes farming and killing that much easier and it shouldnt be too hard for you to kill it yourself but remember to ask permission and its always easier to have your jungler assist you in taking it, often times when doing well you can invade enemy territory and steal there blue with ***itance from team mates, although you dont want to waste too much time away from farm and EXP.

Sion can also easily take on dragon even as AP past level 11 by himself, and at level 6 you can easily be the tank for dragon with help from bottom lane and your jungler, your burst will assure you they dont flash in and smite it and your sustain will allow you to stay on the dragon and keep at full then be able to fight off enemys as they try to stop you.
The final thing to remember when playing Sion is that you are AMAZING at ganking once you get boots of mobility. Force your middle to recall by repeatedly doing high amounts of damage, clear your wave, then run to top or bottom to gank activate your sheild when you are going to go in, ask your support for cv so they dont escape into the bushes and stun the enemy then proceed to 3 hit him with a combo of Q, W and one sheen boosted hit.

Advice:
Engage with Sion, dont try to recover, if they engage you first and you are at the disadvantage without your sheild you probably want to back up unless ofcourse you have your full team with you in which case you want to back up activate sheild then run back in and tear some face. Push With Sion, his steriod gives him plenty of AD and his ultimate gives him alot of AS and his W gives him a one hit minion wave clearing button. Dont play Sion like an AD carry, with this build you should do your damage and if you arent going to kill somebody or survive the next couple of seconds you are going to want to back up activate your sheild and run back in there with your stun staying on champions can get you killed against a good opponent.

The runes I take are sort of standard but vary a bit because of Sions unique abilitys and scaling.
The Runes I take are:
9x Greater Marks of Insight for increased damage
Greater Seal of Replenishment9x Greater Seals of Replenishment to negate your Mana problems
9x Greater Glyphs of Focus to keep your abilitys up when you need them
3x Greater Quintessences of Potency Sion has 1:1 ratios in his two damaging abilitys and AP is amazing on him

Options:
For your marks there really arent any better choices a bit of natural Magic Pen is a must.
For your Seals you could take a number of things, Greater Seals of Potency will increase your damage slightly, Greater Seals of Warding will help you counter Casters in middle, and Greater Seals of Fortitude will make you tankier all around, but with any of these you will encounter mana issues more often early on.
For Glyphs you could also take Greater Glyphs of Potency which would increase damage, Greater Glyphs of Warding which would help you counter Casters early and many others, but I find greater cooldown is very helpful.
For Quintessences you could take Quintessences of Fortitude for some added bulk, Greater Quintessences of Swiftness for increased ganking potential and easier juking, and Greater Quintessences of Insight will help you out, but I find using the Quint spot for AP will greatly increase the amount of times your opponent has to recall.

Really standard AP caster masteries 9/0/21
9 in Offense for added damage and CDR.
21 in Utility for extra EXP, mana regen, buff duration, speed, summoner spell CDR, and abilty CDR.

Once again very standard AP caster Summoner Spells
Flash to get those last hits on champions, initiate with your stun in team fights, and escape ganks.
Ignite to get those last hits on champions and to stop life drain.

Sion has a very cool skill set which is good early game and late game making him very versatile and usefull on any team.

Cryptic Gaze: The easiest stun in the game period, high base damage and an amazing 1:1 scaling shared with his next ability

Deaths Caress: A very sturdy sheild that allows you to farm protected and one shot entire minion waves with the right amount of AP, the high base damage and 1:1 scaling compensate for his Q and W being his only 2 AP damaging abilitys.

Enrage: An awesome damage steroid makes last hitting a breeze and has one of the best passives in the game, every time you kill anything you get points added to your maximum HP PERMANENTLY allows you to be extremely tanky late game with resistances.

Cannibalism: Sions Ultimate, not the best one for an AP carry but is amazing for lane duration, allows you to achieve 100% life steal for a good amount of time and just gives you pretty much a full heal on minion waves and as long as you are behind your sheild you lose no hp so its all gain. This allows you to stay in lane for days and take neutral minions like its nothing.

Feel No Pain: Sions Passive, a very simple damage reduction helps his survivability a little bit especially on people building more AS than pure damage.

Skill Sequence: Start with either your stun or your sheild, if you are facing somebody like Fiddlesticks in the middle for whatever reason who has something that you need to interupt stun is what you want, but more often then not sheild is going to be prefferable, it allows you to last hit, take some hits, and do some damage to the enemy carry all at level 1. Then you want to get stun or sheild depending on what you got first and that will complete the two buttons you will be pressing do damage for the rest of the game. Third you get enrage so you can start stacking health and last hit better and complete your combo better. then you want to work on getting your sheild to caster minion one hitting status while also upgrading your ultimate whenever you can and keeping your Q and E on par because they are both extremely usefull, upgrading either is a good choice which is the worst thing about Sion, do you want to get more health or do more burst damage? I just like to get I think level 3 sheild then level 2 in stun and enrage for utility then max sheild then keep the stun and enrage even untill level 18.

The first couple of items are standard for casters and then you get a little bit different because of Sions unique form of dps.

I start with a Doran's ring for extra damage and extra lane sustain and that will keep me busy in mid for a couple of levels and should allow you to farm all the way up to boots , another Ring , and a couple of health pots and depending on how your lane is going maybe a couple of wardsSight Ward to prevent ganks and control buffs/ dragonsize=40size=40. Then I get one more Ring for a total of 3 rings and boots, after that I move on to a Needlessly large rod to start working on my Rabadon Deathcap . Since Sion is oriented around a mixture of ranged AP damage and a couple of auto attacks Sheen is the perfect next item to build then you can get mobis for ganking but alternitvely you can go any Mixture of Mobis/ NLR/ Sheen depending on how you want to play, Mobis for ganking, NLR for farming with your sheild, and Sheen for burst damage on the enemy in your lane. After that finish your Deathcap and your Lich Bane then you are pretty much done honestly, I almost never get passed this point with Sion I faceroll so hard they surrender very early or we just have pushed into there base and won it, but if you arent done by that point get a Banshees veil to help your sheild stay alive in team fights and stop them from using crowd control methods on you. After that I think a Guardian Angel would be universally good but if somebody on there team is getting particularly fed be it AP or AD just get something to counter them, for attack speed/ AD champs get Frozen Heart / Thornmails (I much prefer Frozen Heart its just more useful on sion) for AP champions, Banshees Veil is already going to be doing wonders but if you really need something more, Abyssal Scepter or FoN will be usefull, Abyssal Scepter will be especially helpful if your team is AP heavy and you need to break some MR and Force of Nature will make you nearly invincible to AP casters without magic pen and will make you even more of a lane sustainer without even needing to use your ultimate. After getting Boots of Mobility / Rabadons Deathcap / Liche Bane / Banshees Veil / Defensive Item it leaves one slot open and on a champion like Sion there are limitless options, you could build more defense, you could build some magic pen like a Void staff or you could build a Deathfires Grasp for added burst and tank killing, the last option you could build would be some sort of AD item which ofcourse would make your more hybrid but depending on the way you play there are a number of AD items that would greatly benefit your late game, like Phantom Dancer to make you faster, Frozen Mallet to make your enemys slower, or Atmas Impaler to capatalize on your increased HP.

Early Game:
+Out farm your enemy
+Zone out your enemy
+Kill your enemy

Mid Game:
+Gank top and bottom
+Push Lanes
+Take buffs and other Neutral minions

Late Game:
+Initiate in team fights
+Kill Towers and Inhibitors
+Tank Baron
+Win the game!

I hope you like the guide and find it useful if you do leave a comment and give me a thumbs up.

Update: I Did this on a whim and after taking a look at another guide its not exactly the same but its pretty simmilar we differ on a couple of points but still... Its way better than mine and im not sure if I should keep this up. If I wanted to upgrade this it would just end up looking more like his and thats just not what I want, to copy somebody else.