This is a build for a 5v5 ap carry Sion.

I'm gonna leave out the basics of how to play the game and hope you know. Go read something if you don't.

I know this build ain't that original or anything... what is these days. This is just my recording of a build people have asked for, generally after I bury my ax somewhere in their cranium. Along with that its just me rambling on about how to play it and why it works. Its mostly a compilation of other **** I read compiled into one place.(redundancy for the win!) That said please enjoy and I hope it helps.

For those who don't know Sion gets a one to one ratio on ap to dmg. So if he has 600 ap he adds that to his stun and shield dmg.

Mastery Choices: Many ppl don’t like my choices but I have tried and re-tryed other builds and I really find this one to compliment my play style best. The extra physical dmg masterys early game helps get kills to stack mej's and the 5% dmg helps hit REALLY hard. If it doesn’t suit your play style then go with a 9/0/21 build instead.

Item build reasoning:

Sapphire Crystal: for stuns and sheen.(all Sion builds should start with this as stuns are key to low lvl ganks on your lane. Even ap sion early ganks are just pure melee and stun.)

HP pots: ...figure it out.

mej's soulstealer: power that scales with your needs.

WHEN PLAYING WITH MEJS REMEMBER YOU NEED STACKS!
DO NOT DIE AFTER GETTING MEJ'S. play safe... not overly so but don't go solo towers or champs. stick together. even if your not getting kills assists give you stacks. heal up often in base if you must. better to run for 3 min than to lose stacks. And remember, don't die ever!
sorc Boots for magic pen... pen = more dmg


DeathCap - ap... more ap... LOTS MORE AP. ahem... sorry.

Void staff: ignore more magic resistance? WINNING!!!

Will of the Ancients: adds ap but more importantly gives sion heals. Fairly big ones too as his stun hits for 1000 + dmg so your gaining 250 or so hp ever time a spell hits.

Lich Bane: I get this late so the ap bonus to your melee hit is hard, this before the rest makes it weak sauce. every time you use an ability you do your ap in dmg next attack so its like another spell at high dmg. Just remember to let those intervening hits get in... that's a lot of dmg to miss.
Watch cool downs on your lich bane you don't want to rush and miss activations.
also this works on buildings... throw up your shield and take a massive chuck outta the tower with your ax. stun minion... smash the tower.

Abilities:
Deaths Caress: shield is priority... main kill weapon and dmg soak. main farming tool too. when you start taking out whole waves of minions in one shield pop and catch yourself giggling and gibbering to yourself you'll know what I'm talking bout. if your not getting tons farm kills you ain't doing this right, or your doing it so right that they surrendered already.

Cryptic Gaze: stun is next, that first ap max stun that surprises the squishy by taking half their life as you prepare to fold em in half with your shield is sweet. you'll get comments... some with profanity.

Enrage: one rank early for a little padding. wipe a wave with shield... ++ hp

Cannibalism: Your ult in an ap sion build is more subtle than in a ad build as it doesn't scale with ap, that being said its still a very useful tool for taking buffs, towers and healing between fights to stay out farming longer.

Runes:
Magic Pen: Your magic penetrates them. More penetration, more rape.
Ap per lvl: right around lvl 10 when you want to start rolling these kick in and help put the hurt on in a big way.

Summoner skills:

Teleport: only way to roll in 5v5. Keeps you in the fight for xp and moolah, and handy for that across board gank as well as tower saves, prolonged pushes, etc. the more I play the more I like this ability. Its versatility is never lost. Helps to support team, towers and always be in the right place in the right time.

Clarity- good for sion... just great. sion's mana pool is small, minuscule even. Use it all game and often. and don't forget to use up your mana before refill. pop up a shield before and then have the mana for stun and 2nd shield and stun.

as you don't have traditional escapes (flash etc) be sure to play smart, staying close towers and watch for ganks, but stun and shield do help a lot in this area. just not so much 1v5, but then nothing really does.

Game Play

Early-
there are two distinct ways to start. if you have a lane partner who is incompetent just harass and farm, farm your *** off.
Lvl 2 or 3 is where to start trying to kill your lane with stun/shield and your team. but farm always farm. also support the other lanes by ganking and holdin em if they need to head back.

Mid-
Go pick up blue aura if its around, red too if no one else is on it. start picking off and ganking with your team but stay on your lane as much as you can unless they other team is feeding you like crazy. farm everywhere you go... blow em up with that shield on the way to ganks... remember that for sion, minion kills = hp, xp and money. Here is where you need to build mej's stacks. Play smart and get in on every kill you can for assists.

Late-

late for sion can be two things, full items rape train... walk around killing everything you see with your team backing you up focusing on towers and inhibs.
most likely the other team will give up right around the time you want the game to go on forever in an endless sea of blood and death.

or starved useless **** sion. just push lanes with team and fall into obscurity. try to pick up stacks and cash as you go... but most likely your hope of using your ax in ungodly ways is gone. sigh. still you can be a solid pusher and if you haven't fed can keep a respectable score.

Other hints and tips

since your shield has a both a time after activated before you can pop it(4sec) and cool down a hit and run strategy is great for sion.

a surprising number of players will chase you feeling like they need to pay you back for taking out half their life... only to realize you can do it again.

kill stupid players, great ways are... they stick around at half health... sion can kill that, got a lane partner? gank That ****!
low health char chasing you down because you even lower? pop up shield smack em around while shield absorbs dmg then pop or just stun and blast em with the shield.

Your stun is a very powerful tool so don't waste it. If there are characters that need ccing (kat in her ult for example) make sure you saved your stun to deal with em.

Fear is your friend. when two players see you coming round the tower with shield raised and run like cowards it does good for your soul. use your intimidation. control areas with nothing but stark raving terror of your stun shield combo. just walk around and people will give way, but use it wisely because that kinda respect can make you get a god complex and leads to foolish risks.

Here is what not to do!
http://www.youtube.com/watch?v=qxmwGq15v-U

And as always, gl hf.