I can't believe the number of people that waste sion using him as AP. This version of sion can easily carry games, and if the opposing team does decide to focuss him, it usually won't matter as you will have enough health and lifesteal that you can just take it. North of 5k health is plain scary.

When you take into account enrage and atmas this sion will have over about 250 DMG at 5k health. That is with only a single item, boosting this is really not important. What you need is attack speed, which comes from your berserkers and starks fervor.

My full item order:

Vamp sceptre -> Emblem of Valor -> Berserker Greaves -> Starks Fervor -> Atmas -> Frost Hammer -> Zeal -> Wamrogs Armour -> Phantom Dancer

A nice thing to note is that your slow from the hammer combined with boost from the dancer means that once you get into a squisy, he won't be getting away.

You can solo dragon at lv11 with cannibalism or lv14 without.

The Frost hammer + phantom dancer + stun makes you a fantastic chaser which is why I use them. If your playing 3v3, once you have your frost hammer you can usually solo their whole team aslong as they haven't been fed, i'm not joking.

An alternative against a caster heavy team would be to grab a FoN instead of the phantom dancer or against a team with a lot of physical carries a thornmail is a good choice replacement.

Sion is terrible early game on his own. He needs a good lane partner as all he has is his stun. So he is stuck last hitting minions. What you can do however, is lane with someone like pantheon and repeatedly slay enemy champs in your lane by abusing your stun. If your good enough to do this then sion will literally dominate the game. Because if you get fed there are literally no champs that can take you using this build.

Once you get to 6 you should start to gank mid, your stun is fantastic for doing this, try to sneak off between minion waves so it looks like your going to ambush them in your lane when in fact you've gone off to gank mid.

In team fights you should go in first, stun a squishy, activate your shield and ult and batter it to the ground. You shouldn't have to worry about getting focussed. Aslong as you make sure you go in when your team does then the enemy will either have to focuss you which will cause them to sink about 6-7k DMG into you thanks to your life steal and shield or let their squishy get nuked. If they do choose to focuss you then your team should manage to roll over them with ease due to the amount of damage the enemy is being forced to put into you.

A nice thing to note is that your slow from the hammer combined with boost from the dancer means that once you get into a squishy, he won't be getting away.

Protip: if the enemy has someone like fiddlesticks or Nunu on their team save your stun until they ult so you can negate it.