As a little introduction before we jump straight in, I'll just share a few words of wisdom with you. I can honestly say, I believe Sion is THE most overpowered and overlooked champion in the league. I very rarely finish with a negative KD, and the main reason for it usually is we are either trolling ( ^^ ) or the enemy team is something super special. This is a guide that I believe to be completely well rounded and works 99.999% of the time. Although, It's not often I get that far through the item list before the end of the game too! But Just so you know. IT'S NOT A SION BIBLE! Just a nice run through of how I do things =D

PROS

Super harass
Very strong throughout the game
Super high AP ratio on DC and CG
Can backdoor even played as AP!
IMBA shield
Super stun on low CD
Works in most team combinations
BIG, GREEN, KILLIN MACHINE! (And he has a massive orange weapon :D )

CONS

Mana hungry early on
Is easily kited sometimes
And....Erm......There are no more CONS!

As you can see, my rune choices are FAIRLY common amongst AP nukes, BUT the most powerful for Sion and for securing first blood.

Greater Mark of Magic Penetration = Means you will rip through those tanks like a hot knife through butter, always a must have!

Greater Seal of Cooldown Reduction = CDR is the most important stat (other than AP OFC) on Sion, as you will need to be as close to the 40% cap as you can. The two spells you will be spamming (DC and CG) will cause complete devastation to the opposite team... This is your nuke stat.

Greater Glyph of Ability Power = Will help you net that first blood, and a quick recall for Sapphire Crystal will only let you net more kills and farm in the oncoming minion waves.

Greater Quintessence of Ability Power = As the above states... Early BOOM! WTF!? potential.

Here we have the generic tanky AD runes, I have gone with the Greater Quintessence of Desolation, I tried the pure AD runes on him, he hits a tonne harder early game, but trails off after you get infinity edge, as the damage is purely hitting their armour, no surpassing.
runes=http://www.mobafire.com/league-of-legends/rune-page-planner#&rune=273:3-263:9-262:9-300:9
However, I have gone with the flat armour/magic resist glyphs and seals. These I thought helped out alot at the first Windmill battle. Especially as your main job is to target, stun, attack, exhaust, so getting into melee ranged at this stage is not only needed, but is your only way to deal damage.

I have tested next to all summoner spells for effectiveness, and these are my conclusions...
RECOMMENDED
Ghost = Mobility, whether it be chasing or running away (not like you will have to that much), Ghost has it covered. And with the mastery Summoner's Wrath you will hit 35% movespeed!

Surge = One of the new Summoner spells to take place of Rally. And thank GOD they did bring this in. It can help in so many ways! Generally used for entering team fights to get more firepower in your spells and to bump up the ferocity of Cannibalism. But the 40% increase in attack speed is wicked. This will help you go mental on those pesky nexus turrets! I try to use Cannibalism every time I pop this spell, to get the best effect possible.

USEFUL

Ignite = I used to use this spell accompanying Ghost, but since the recent addition of Surge, I don't think it is as brilliant as it used to be.

Flash = I have tried this spell a few times, but since the nerf, I feel this has nothing on Ghost .... 35% movespeed is too good.

Clarity = You really shouldn't spam your spells that much ^^ But for beginners, or people getting to grips, this can save your bacon early game

JUST....NO!

Do I have to spell them out?

icon=Exhaust size=55 icon=Ghost size=55 icon=Ignite size=55
These Summoner spells work beautifully with AD Sion, Exhaust can be used for a get away or for keeping people close to you, Ghost, again for the same thing. And Ignite is equally as brilliant, As I stated before, Sion gets kited so very easily, so Ignite is always good for helping get that last long range hit on someone, or if you are duelling another Sion, or tryndamere, or any auto attacker in general, you will notice that they only get health from lifesteal, hence the reduction on health regen with ignite on someone ;D

As you can see above, the masteries chosen improve AP, CDR, Magic Pen, Movespeed and Mana pool. These work THROUGHOUT the game, whether it be early game, mid game or late game.

Mental Force = AP...Even more nuke

Summoner's Wrath = Will help improve your summoner spells.

Sorcery = Will add to CDR, nudging you ever slightly closer to the CDR cap.

Arcane Knowledge = Will accompany nicely your marks to help get closer to true damage on squishies and helping to shred the tanks magic resist.

Havoc = Will help very slightly in early game (not enough to particularly notice) but will come into full effect as the game goes on.

Blast = Same thing as havoc, not nearly enough to notice early game...Mainly to help your late game nuke.

Archmage = DITTO! will help you early game, but when reaching Rabadon's Deathcap .... The enemy team won't know what hit 'em!

Executioner = Will help your attacks bring your enemies to their knee's! Always good for that escaping Akali !

Expanded Mind = Mainly to help your early game harass and spamming.

Good Hands = The only other 'useful' mastery in the first line.

Swiftness = Help mobility around the map... Who doesn't want that?! Useful in too many ways to miss out.

Runic Affinity = More time with neutral monster buffs, Blessing of the Lizard Elder, Crest of the Ancient Golem and Exalted with Baron Nashor. This is beautiful around mid when roaming and running in and out of small champion fights are a regular occurrence, Just be sure NOT to die with these buffs affecting you, as the enemy will inhibit this buff.

As far as AD Sion spans, his mastery build is very common and is the stereotypical AD nuke build. 21/0/9 means Sion gets the needed steroids for both AD and speed.

As you can see above, the generic crit/AD masteries have been chosen. The main problem for Sion in Dominion is getting kited, this is why I have gone for the utility tree, with this I am able to get points into Runic Affinity to make the speed shrine buff longer, and points into Swiftness to help out with this major problem.

These go without saying! And I'll explain along the way.

Doran's Ring = MP5, AP and health... First blood on it's way! Survivability and nuke in a ring.

Sapphire Crystal = Building to catalyst and helps your spamming and harass.

Ruby Crystal = Building to catalyst, helps survivability... Brilliant!

Catalyst the Protector = Mana, Health and regen whilst levelling, this item is perfect for early game farming... And also builds towards Rod of Ages .

Boots = Movespeed building to your main boots. Does what it says on the tin.

Sorcerer's Shoes = Magic pen is always perfect for any champ who uses AP! And faster movespeed.... <3

Rod of Ages = I can honestly not show you in words my love for this OP item... Health, mana and AP... Increasing over time! The stats are so good to start with!

Kage's Lucky Pick = I always build this first for DFG, gives you the GP5 and AP to go into fights, and also in those awkward moments hovering around mid, you are generating more and more gold... TIME IS MONEY, FRIEND!

Fiendish Codex = CDR, MP5 and AP. This item in itself has a pretty big impact on Sion, and ofcourse building into DFG, your third nuke as Sion.

Deathfire Grasp = CDR, MP5 and AP... BUT, this item will shape your Sion into a serial killer, adding the missing link to your combo.. More to come on this later in the guide.

Fiendish Codex = Our friend is back! and SO close to the CDR cap, too!

Blasting Wand = AP. Pure and simple.

Morello's Evil Tome = Tonnes of AP, CDR and MP5... And we hit our goal... 40% CDR, get ready to cause devastation.

Needlessly Large Rod = Again, a tonne of AP and building into The Deathcap.

Blasting Wand = Yep.

Rabadon's Deathcap = Get ready to PWN!! 30% increase in AP?!!?! The stats are GODLIKE!

Catalyst the Protector = Building into Banshee's.... Like we even need this at this point xD

Banshee's Veil = TAKE NO DAMAGE EVER AGAIN! xDD

http://www.mobafire.com/league-of-legends/item-purchase-planner#&item=66-37-82-52-17-16-46-22-66-51-41-52-130

This is the part where you guys get your say, too! PM me or place a comment below with your item and reasons for wanting it being placed here. Credit will go to the people with the best explanation mixed with first come first served.

OKAY!!!!!! I know half of you here are thinking, 'Where the f**k is Sheen/ Lich Bane?!'. Well, here is my reasoning. I have definitely tried these in my builds, and I can honestly say that it has nothing on Morello's Evil Tome. The stats on this item trump Lich Bane so much for me to say that I personally feel Lich Bane is rubbish on Sion. NOW, at this point I can see you thinking that I am bias towards the Tome, but there is an argument for Lich Bane. I can see this would be very handy when battling with tanks stacking heavy magic resist. This, I'm afraid, will have to be used in place of Banshee's Veil. I mean Lich Bane has a bit of magic resist, but in order for it to be truly worth while, it will have to come at the end of this build. The CDR is too much to miss out on in the mid game team fights.

Sion's moves can wreck teams if used wisely, and the ability to solo duel virtually any champion in the league is just ludicrous! And here are the moves that make all of this possible!

This move when capped on CDR will mean you can get off one of these bad boys every 3.8 seconds! And the 1.5 second stun will cause your enemies to rip their hair out! This harass is just too good, and when paired in a combo with Death's Caress can bring down an enemy in a snap. This move late game can often determine the tides of battle, just pick out an enemy carry, stun, POUNCE, watch 'em QQ in all! But this also makes chasing down a loose enemy so incredibly simple paired with Ghost and a DC.

This move I have decided to max out first due to numerous reasons. Firstly, it has a bigger base damage value compared with CG, making the early game farm to incredibly easy. Secondly, the shield absorb on this beauty with epic! Absorbing the same amount as it does in damage. Third....ly, in that possibility of the enemy jungler moving passed your river ward(s), you will be safe in the knowledge that you have a lovely stun and a shield that will make the enemy jungler rage and QQ at his team mates.

I have placed a point in this skill third due to a couple of things, Mainly due to the damage bonus you get on your basic attacks. And a little less importantly, but equally as useful, you will get an extra +1 health PERMANENTLY per kill you gain. When in late game and you run into a stream of minions and blow your shield, the health increase will make sure you can be there for your team fights, and ignoring the stereotype that all AP champions are squishy.

This spell isn't as important for you in the whole team fight scenario, BUT this spell in combo with Surge will EASILY make you able to backdoor. Yes, I said it, backdoor, and not only inhibitors. I mean ENTIRE TURRETS! In combo with your shield, tanking a turret by yourself is a piece of cake. The lifesteal on it will help in teamfights ofcourse, but it's not a disaster if you don't have it at the start of a team fight.

The ability build I have chosen gives you instant satisfaction on damage, even at level 1, enrage gives you a whole 25 damage! Thats the same as a 975 gold Pickaxe!! So, I decided to max Enrage first, as the damage steroid you get is just mental. I have maxed out Death's Caress next, this is for the pure fact that at level 5, the shield absorbs 300 damage! without AP! thats just crazy for someone who isnt stacking AP, making your duelling mental, and god knows duelling in dominion is a regular occurence. I max Cryptic Gaze LAST because, the only thing it gets through getting levelled is damage, so the stun doesnt last longer, the cooldown on it doesnt shorten, so why max it if you arent going AP? You only need it to keep people in their place. and, most importantly, Cannibalism, this is, being bias, THE best auto attack steroid on the game. 100% life steal! 50% extra attack speed, AND it heals your teammates around you for 50% of the damage you put out, this makes you one, if not, THE biggest asset to a team you can get in dominion.

Sion, in early game is immense. First blood nearly always occurs with me in lane. With Sion being an overlooked champion, there are a few people who still know very little about what Sion's abilities. This is a GREAT advantage. Throw your stun only when you co-ordinate with your team mate, other wise the possibility of first blood could pass. Use it wisely, both offensive and defensive.

This is where Sion becomes a weapon of mass destruction. GET FED! This will make your late game so much more effective. Even one item from the build makes a massive difference. By the time you get to level 8-9 you should have the turret down and have the enemy feeling de-moralised. Mid should be the next turret down and as a team you should be pushing. If top still hasn't got their turret down, you need to be up their making your presence felt and putting immense pressure on the enemy hugging their turret. Try your hardest to get a kill but don't get complacent, don't make silly mistakes. DON'T DIE!

KILL, KILL, KILL! By now you should have at least a Blasting Wand into Rabadon's Deathcap by now and should be wreaking havoc. Late game Sion's power has fallen in accordance with all the other champions. So pick your battle wisely, don't underestimate a Tryndamere! Don't think you can go into any battle what-so-ever and get a triple kill. It won't happen unless you are co-ordinated with your team and you know all the repercussions of a fight and weigh up whether or not you believe you will win. After all, just one man alive won't keep every turret alive. Go and win for your team! GOOD LUCK!

In this guide you will find quite a few abbreviations, here's a key for you so you don't get confused.

AP = Ability Power
MP5 = Mana regeneration per 5 seconds
HP = Hit Points, or Health
DFG =

Deathfire Grasp
GP5 = Gold per 5 seconds
CDR = Cooldown Reduction
HP5 = Health regeneration per 5 seconds
CG = Cryptic Gaze
DC = Death's Caress
IMBA = Imbalanced
OP = Over powered
CD = Cool down

There we have it guys.... My Sion guide is now finished! I hope this gave you a lot of ideas on how to play Sion in ranked and normal 5v5 and 3v3's. This is 100% the way I play Sion. No changes added ^_^ I have put quite a lot of effort in this guide and I believe after every patch, there is something new to be added, and that is my guarantee.

If you want to PM me in game, I play EU West - Summoner name DEIRU.

Thanks for reading guys, and have fun with Sion and his undead ways!

TO DO LIST

To constantly update new patch notes on Sion
Screenshots to be added A.S.A.P
POSSIBLY an AD build

Updates

16/02/2012 - Took a point out of

Mental Force and put it into [Runic Affinity]]. I figured 1 more AP isn't as effective as having extra time on blue, losing a significant amount of time out of CDR and insane MP5.
16/02/2012 - Added Runic Affinity tooltip to Masteries section.
24/02/2012 - Added AD dominion build, midway through completion.