Hello,I'm Sunfire230 from EUW server.
I made this guide because i played ad sion nowadays and i liked the raping:D
So,i this guide i'm gonna show you some builds/tips for true Sion play.(i've written this cause i think AP sion is viable only in early/mid game.)
He's a great tanky carry cause he does not need any tank items thanks to: Enrage farming.

Easy part:You will need lots of armor penetration because you deal physical damage and not sure to buy armor pen items. Last Whisper/ Youmuu's Ghostblade
- Greater mark of Desolationx9
- Greater quintessence of desolationx3


And because u wont buy much resistance:
- Greater Seal of Armorx9
- Greater Glyph of Magic Resistx9

The masteries are simple, you need everything for physical damage in offense,the last 9points to defense: Resistance and Hardiness gives minimal defenses+ Vigor for lane sustain.

So lets explain items.
First,the generic build:
-Start with Vampiric Scepter --- this will give enough sustain near your high damage.
-get Boots --- minimal movement speed boost you will need.
- Zeal --- your first dps item,give movespeed to chase attack speed for fights and minimal crit to show your enemies who you are!
-Lets finish Berserkers greaves --- Ice attack speed and further movement boost.
-Farm to Phantom Dancer --- Doubles [[zeal]'s effectiveness.
-Get Infinity Edge for further crit increase.
-In early-late game you wil have much HP thanks to Enrage so Atma's Impaler will give a nice damage boost and a bit physical defense
-For 5th items i like to buy a Spirit Visage so i can increase effectiveness of my ult+MR and CD reduction comes well.
-Last items is Bloodthirster --- U will need that lifesteal if your ult is on Cd.

Situational Items:
- Wit's End vs high magic magic damage or Thornmail/ Rammus
- Quicksilver Sash:vs suppressers like Warwick and Malzahar
- Black Cleaver:against really high armor
- Frozen Mallet for chasing.

You will max: Enrage first great damage and you will need the farmed health.
Secondary the Death's Caress which gives minimal defense in fights/duels.
The last will be Cryptic Gaze because it wont hit hard and the cd reduction if not much/lvl.
And of course take on Cannibalism whenever you can.

I prefer Ghost+ Exhaust because u will chase many people,and Sion's main weakness is the missing of gap closer skill.Secondary:this mastery: Summoner's Wrath

Others:
Flash:nice,you can open and get running opponents or juke through walls.
Ignite:vs healing/sustain chars or finishing low hp champs.
Teleport:if solo top and you have to leave your lane itS nice
Cleanse:against massive cc.
I havent mentioned many,i think i dont have to explain.

Pros:
-Best late game hero(near Veigar and Nasus
-awesome damage
-great sustain
-powerful 1v1 and teamfighter
-tanky without tank items.

Cons:
-needs much farm
- Ignite is big weakness
-no gap closer

Early game:focus on creep kills,in melee range attack sometimes the enemy for harass,care for ganks.After lvl 6 you can 1v2 vs not tanky targets
Mid game:coninue farming,be aggressive,push turrets.
Late game:farm moar!!!!! and be in every teamfight casue Cannibalism can save your team!

Sion is a great character with high damage in full game and tanky without anything.
Be careful don't attack the tank:D

Thanks for readin:
Sunfire230-EUW