Hi,this is my first guide so be gentle~
I started playing Sion after being raped in ranked games by AP Sions.Most think "He only has two spells that scale with AP,he is more viable played AD".Well,don't be so sure.
AD Sion can only focus 1 person and can be melted(not to mention easy countered) very easy while AP Sion can do a simple combo to major damage all 5 and kill one instantly.

Pros Cons

Very good farmer with Shield Very squeeshy early game

Great disabler Mana problems early

Extremely good ganker Can be countered easy

For runes i get Mpen Marks(use it on any AP caster,it's SO MUCH BETTER then AP marks)
Mana regen per level seals(you need that mana,no,SRRSLY)
ABility per level glyphs(it can boost you up alot if you're underfed)
Ability power flat quints(it self explains i guess)

9/0/21-The 15%pen and bonus movement speed so damn good :)

Always start with a Doran ring,all the bonuses are esential."But what about health regen,what if we get owned mid?"Explained in gameplay section.
After i take boots and another Doran(if not enough then buy boots and 2 health pots if ur getting harassed)
Next big step farming for the needless and mobility.Why mobility?Because Sion is a beast ganker and you need that boost of speed so that your enemy laner doesn't have time to call miss or if you went rambo and died early and getting back to a team fight.
After getting rabba and lich you pretty much rhape almost any1.
Zhonya for the AP and Active(Q-W-Zhonya-Q-W-PROFIT!)
Abyssal is epic the bonus AP and Aura are simply divine for both you and your team.
Last but not least Deathfire is the cherry on top,if it's not finished then you need something to really finish ppl off.

I have seen alot of people maxin Q first.DON'T DO THAT!
Q stuns the target for 1.5 seconds at all levels so why max it :|
By following this guide you max W and it is your main farming skill.
1 level in Q at lv 2 and you leave it there till 8.
1 level in R at lv 6 and leave it there till the last 2 lvs,you are only going to use it after battling an enemy and making your hp back.
Max my ulty last...WHAT?Yea you heard it,you need the additional hp that E gives so maxing it is a priority to R.

Flash-Best Summoner spell ever!!!!Whether you run away or flash-W it's just amazing.Sion's Bread n Butter Summoner.
Ignite-Extreme Damage.Needed to finish off enemies at early levels.
Ghost instead of Ignite-Great spell but i feel that Ignite-Flash is much much better on Sion then Ghost-Flash.
Teleport-Good choice in some games.Good ganks and you can be back on your lane in no time!But i feel that it is not needed since your mobility boots make you fast enough.
Other spells are not needed.

Oh yes,farming <3.
Always keep your E activated because it works with your W and gives you that so need extra hp <3.
If you cannot afford to last hit because of a good enemy laner just use your shield and pop it!:)
If an enemy is near try to stun and pop it so that it hits both him and the creeps but mainly just poke him so that he starts to fear you and stays back.Focus on farming!

Early game:Just try to poke your enemy enough as to leave you farm.
Mid game:After getting mobility and 1xx AP go gank top or bot because it is an almost guaranteed kill(if your teammates don't suck that is)
Late game:By now you should have HP and with your passive you be quite tanky.Combo is:Q-Deathfire-W-Zhonya-R-Q-W)
And don't forget and this applies for the entire game,DO NOT GO FIRST IN TEAMFIGHTS for you will be focused and your shield will probably be wasted(fear,silence or stuns can prevent you from exploding it).

Thank you for visiting my guide and hope it helped you even just a little :)