I realize that many people like to play attack damage Sion, but I believe that AP Sion is more useful. His 1.0 AP ratio is there for a reason, not to mention with the right items that his cool downs are only a couple seconds at most. Death's Caress is Sion's best skill and makes him an incredible threat when a team battle starts with everyone within a short range of each other. We have all seen an AP Sion who we have feared going near.

I like to go with magic penetration runes marks because Sion already has great AP ratios and Mana regen seals are very helpful early game because his skills all cost around 100 mana. Cool down reduction glyph's are pretty much handy on every mage, without their abilities they can't really do anything.

Starting out with a mana crystal is best because of his mana issues and then building a sheen out of it maximizes early game damage which can get you fed easily off of the squishes (hopefully you're facing squishy people). I think that Mercury's Treads are the best movement option because Sion needs to be able to shoot his stun at the enemy carry and or fleeing enemies for the easy kill. Rabadon's Deathcap is a pretty straight forward item, you need the high AP for some even better nuking. Finishing the sheen into a Lich Bane gives you a better attack after your stun, magic resist, more AP, and a handy speed boost. Banshees for the sustainability of more mana, hp, and magic resist. Zhonya's Hourglass because of the armor and damage along with a great Unique Active that can save your life. With a Rylai's Crystal Scepter to top it off with the passive slow, ap, and health making you a great mage tank.

Death's Caress and Cryptic Gaze are by far the most useful spells so I max them first, mainly Death's Caress because it does great damage and gives a great shield. I choose to put one in Enrage because of the farming bonus of more health when you use Death's Caress on the minion wave. Pretty much when attacking people, shoot your Cryptic Gaze to stun the target, activate Death's Caress, hit him with the bonus Lich Bane damage then explode Death's Caress and the target should be severely damaged. His ultimate is Cannibalism which I tend to use as getting health back from a minion wave, when a melee dps is hitting me for health back, and or killing blue buff to make it go faster.

I go with Ghost and Ignite because Sion with ghost is very scary when chasing and or jumping a target to get off that stun and then proceed to kill the enemy. Ignite is a great spell because you never know when a target could get away with around 100 health and that really pisses me off.

Sion can pretty much lead his team late game to where ever they need to be and stun one target with Cryptic Gaze and finish him off very fast and continue to do so while always targeting the enemy carries when possible. Killing the carry quick and easily gives a huge advantage to the team.

Sion's Death's Caress is a great farming spell, just head into the minion wave and activate it killing the mage minions and damaging the warrior minions to the hp of one auto attack while increasing your max hp because of enrage.