This build is about a Ability Power and a Little tanky Sion.
It's my first build so I hope you like it.

Greater Mark of Insight.
Whit it you will hit harder whit your spell. Like your stun an your shield.
Greater Mark of Replenishment.
It will be useful for your mana. At the begining of games you will have difficulty whit your mana, but whit these Marks your problem will be resolved.
Greater Glyph of Potency.
This will Really help you at early game. It gives you some Ability power.
Greater Quintessence of Potency.
Gives you 15 more AP in the beginning of the game. So you hit +15 on your stun and when your armor explode.

I chose to put 9/0/21 like most of AP Characters.

But If you want you can chose to put your points in defense, if you want to be more tanky, but it is not recommended for this build.

Don't put your 21 points in Offense.

Recommended

I chose doran's Ring at the beginning of the game for the mana regen who is really important for Sion, The AP so he attacks harder and the health.


Why these boots? I chose these boots for the magic penetration. Your spell will hit harder. And thats good :D


This item is really good for sion it boost all he needs. First is mana problem will probably be solved after bying this item. It gaves 525 mana. Second it gaves a tasty 450 health bonus and a yummy 60 Ability power so you hit harder! You need to by this first because you can stack it to have more bonus.


Ok now why take Rylai's Crystal Scepter? It gaves some Health and Ability Power. So this is perfect for Sion !


When you get this your beginning to hit so hard! This item gives 140 AP.. And +30% of your AP. You have to get this item.


First it gaves AP, Second it gaves you magic resist and reduces magic resist of ennemies..

For Situation

Whit this item you have no more mana problem. And gives a some AP .. Not so bad for sion.

Some Sion use this replacing another item. Personally I don't really like it but you can try.

A good item for sion but it cost a lot and I prefer other items..

Another good item for Sion that you can chose.

I start whit stunso it help in ganks at the beginning. Then i boost my stun and armor. At level 4 i take and activate it so i gain +1 at my max Health for each minion killed.

Everytime i Can take my ULT . I get it.

Boost your stun and armor as much as you can.

Recommended

A very good summoner spell.
It help to run out of ganks. And to run faster to get a kill.

I take this because you can get kill some ennemy who were supose to run out. You know when your ennemy escape whit 100 Hp.. Ignite can get the kill.

Other Good Spells

A good summoner spell. It slowes the ennemy so you have more time to get him down.

Can be use to replace Ghost.

A good summoners spell in 5vs5. You can teleport to places.

Pros

Stun very useful in TeamFights.
Great Nuke damage
Pretty good Ultimate
Tanky

Cons

Mana problem.
Nuke damage low when armor destroy.

Chek these vids. It can help you.

Sion Tutorial Commentary

A normal Game Whit Sion Pt.1

A normal Game Whit Sion Pt.2

I hope it helps. Plz let a Comment. Say it if you see something wrong thanks you.