Hello this is my first guide here on MobaFire. When I started playing I didn't know what char to choose. and then in one week I got random Sion. That was when I understood this guy is the perfect character for me. And the knowledge I am here to give you is yours to take if you can.
NOTE: BEFORE YOU DOWNVOTE: I am not lvl 20 yet so I don't have runes, when I get to that I will add runes to guide. Try build before downvoting.

pros
-Great nuker late game
-great hp pool due to Enrage
-explosive shield
-awesome damage stun
cons
-very low mana pool in begining
-squishy in beguining of games

ok so the masteries (I am not lvl 30 can't try them but that is what I am trying to obtain) are the 9/0/21 for obvious reasons that I think don't need much explaining.
Sion has mana so I use utility tree. and the ofensiver reduces your cooldowns a litle and gives Mpen and litle crit chance.

In here I will explain the skills of Sion.
: this passive is not very good nor bad. it has a chance of blocking a good part of incoming damage but it does not always trigger. I don't know how many times I did not die because this saved me.
: awesome stun with 1:1 AP ratio what could you ask more than this?
: the cherry on top of the cake: a shield that increases more damage as you get more AP and also damages more with AP. simply awesome.
: Although you are not AD this skill can be good cause it gives you a decent AD boost and the extra health when you kill creeps.
: Excellent for bringing down towers and recovering from low health situations.

The Item build is situational. Doing ALWAYS the same guide is not advisable. however there are some items that cannot be forgotten. these are called core items.
My core items for this build are: Doran Ring Rod of Ages and Morello's Evil Tome

NOTE:

Doran's Ring will be futurely replaced with Banshee's Veil. I did not include boots because if other team has loads of CC (Crowd Control) they Mercury's Treads will be more usefull
let's start then

Early Game

Pretty obvius for a start. I mean look at stats. gives you a litle boost at hp + mana regen and even 15 ap. the ap might not seem much but it is certainly usefull.
an alternative to this would be buying an Amplifying Tome and 1 Health Potion

OR

If other team has loads of CC pick up Mercs. Otherwise go for the Mpen.

Mid Game

helps with mana problems. no more holding back on stun! plus the hp boost is always welcome, and the AP is quite good after some minutes



cd reduction ap and mana regen. this item is AWESOME for Sion that litle CDR might not seem much but it actualy helps a bit.



70 AP and 40% MPen. This means Tanks get more penetrated =) you can take Abyssal Mask instead that gives some nice MR.

Late Game

At this stage you must be feeling awesome with the stun + shield combo. its time to pump the damage to the MAX


pure raw AP to feed your stun and shield. the best is that it increases your total AP by 30%


If you reach this point of the build you are either in a really long game or are extremely good at farming and ganking. I like adding some survivability with Banshee's but you might take Guardian Angel or if you want to further increase your AP take Zhonyas's Hourglass.

Credits to Cháncellor his guide has been my starting point for my guide, and also the runes layout until I can start testing.
This guide is still very small but I think that with time I will actually learn decent BB code and expand it as my game experience increases.
Constructive criticism is welcome

October 4th 2011: Added item section, corrected some tipos