Hello, this is my first guide for league of legends. Sion is an under-used champion that is commonly know for his carry (mainly cause of his life-steal ult.) In here, I will discuss a unique build for sion, the ap off-tank.

This rune set up will either increase sion's effectiveness or strengthen his weakpoints. I started off choosing Greater Marks of Destruction. This gives you some magic penetration to make your abilities more effective. At the same time, this will give armor penetration; this will allow you to deal with other tanks better and will bring at more damage once you get your atma's impaler. Next is the Greater Seals of Endurance. This will also give you more damage with the atma's impaler and it gives you more health (which is always nice.) Last is the Greater Glyphs of Shielding. Without this, Sion will be weak towards ap carrys (especially veigar and his ult of ap destruction.) This gives you an extra 24.3 magic resist at level 18 (this has saved me one too many times.)

Since the defences of Sion were handled with his runes and items, the masteries are focused on increasing the ap half of Sion. I put 21 in combat and 9 in utility. This will give damage, ability power, cooldown reduction, mana, mana regen, and some movement speed and spell vamp. Lastly this will help with your ignite and flash spells. You can put your slots on the AD side of the combat tree. If this is done, you will lose some of you over effectiveness with your abilities, but will increase the attack damage you give out (especially with atma's.)

With your item build, it is important to build your ninja's tabi first, but which way you build it isn't as important. I just recommend to start with the cloth armor mainly because you'll find yourself soloing often in either top or mid and possibly bot if your ally is feeding your opponents. Afterwards, you will either want to get you Chalice of Harmony or start building on your Thornmail. This depends on what your enemies are and what they are building on. Another important thing to build on is the Spirit Visage. This is incredible item that gives you health, magic resist, health and mana regeneration, cooldown reduction, and some spell vamp. Next you upgrade your Chalice into Athena's Holy Grail. This and next two items are built last because the Sion works well late game. This gives you ability power and a boost in mana regen, cooldown, and magic resist. A interesting feature from this item gives you mana when you get a kill or assist. This increases your survivalbility late-game because sion needs mana to last. After that comes the Atma's Impaler. This is an awesome it that increases your damage based off your health (and Sion can get literally infinite health.) Aside from that, it gives crit strike chance and some armor. The last item you get is the Lich Bane. This finishes your ability power and your magic resist and gives you increased movement speed. The most important thing about Lich Bane is it's passive. When you use an ability, your next attack is increased based off your ability power (that makes your attack damage doubled every two seconds.)
You can change the Atma's with the Sunfire cape if you plan on being more of a tank than an off-tank, but you lose a lot of overall damage from it.

The skill sequence is pretty self-explanatory. You level up your Death's Caress to give you a good shield and maximize your damage with it. Afterward you level your Cryptic Gaze to make the stun last longer and increase the damage. Last to level up is Enrage. The damage it gives isn't important until late-game when your in the middle of five champions.
The Cannibalism ability is always important to get when given the chance. Use in when your low on health from killing minions or during the intense team battle with the extremely fed darius. With the cooldown reduction, you get your ult back real fast so don't hesistate to use it if your low on health (don't want to get ganked and die because you weren't prepared.)

Ignite- Prevents others from using you as a health steal dummy; if the enemy is low on health, use it to kill the champion. It has a higher range than any of your abilities and it has a reasonable cooldown.

Exhaust- This is always my choice for a spell. It helps in 1v1s and will help you chase or escape enemies