So this is my first build guide, I hope you give it a shot and enjoy playing Sion as much as I do. This build plays well if your style of play consists of all out assaults and never say die mentality. Some call it kamikaze style, I call it honorable aggression! Either way, I think that if you follow this build you find that Sion really is an OP character as long as you can avoid a complete team of cc Champions. Enjoy!

mark of armor penetration + quintessence of armor penetrationI think that playing Sion in this attack damage mode you should amplify that with 9 mark of Armor Penetrations and 3 quintessence of armor penetration to beat down on those tank and ADC Champs. I am working on buying a set of 9 Mark of Attack Damage and 3 Quintessence of Attack Damage to try also with this build. If you have them already I can imagine great things in your future :)
+ Since you will likely get a lot of attention with the dps you will unleash I also recommend 9 Glyph of Magic Resists and 9 Seal of Armors. I like taking all flats because Sion benifits more from the boost at low levels since he is Godlike lategame.

The first item you work for is the Vampiric Scepter for two reasons. It gives you a nice early boost to attack damage and negates the health cost of Enrage which as I mentioned, should be ON AT ALL TIMES. Start with the Long Sword and 2 Health Potions, farm up some gold for you next stop into town. Upgrade that thing ASAP and buy some stinkin Boots while your at it.

Start working on your core items by building the Stinger. A quick boost in attack speed will help you farm better to save up that gold quicker for your next item.

Next items you should buy are the Zeals, this will add more attack speed, crit chance, and some bonus to movement speed as well. Save up another 1175.

Now that you can build that second Zeal. Those 3 items will net +76% to your attack speed, +20% critical chance, 10 movement speed and 10% Cooldown reduction.

Don't forget to upgrade your boots of speed to Ninja Tabi Now you can start slaying all the champions around you, unless you get slammed with some cc your Ultimate Cannibalism should make you virtually unkillable. Farm up on those Champions, kill the dragon if you have a chance, and keep killing creeps...FARM FARM GANK FARM FARM GANK.

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Everytime you hit about the 1500 gold range you should start heading to town to start upgrading those Zeals and your Stinger. The Phantom Dancer is probably the first one I get to bump that crit chance up to 40%, it will cost you 1625 gold. Then turn the Stinger into a Zephyr This is where the twist comes in. Up to this point you have been focused on attack speed and crit chance. Now that you have killed all there champions, likely 2-3 of them at a time they will start to focus on you first. Build a Spirit Visage from scratch. This item will boost your hp a little, give you that needed magic resist, and the UNIQUE passive for it will add to your lifesteal.

With these items you now have +118% attack speed, +30% cooldown reduction, +20% movement speed, +40% critical chance for offense, then 62 magic resist and 37 armor for defense, not to mention the passives block 10% of physical damage, tenacity, and sions Feel No Pain passive ability. That should leave you free to kill just about anyone who opposes you.

Now, its time to really get nasty, since your spirit visage helped boost your regen and lifestealing you can sell the Vampiric Scepter to make room for a Nashor's Tooth It can't hurt to increase your attack speed again from +118 to +168% and while you are at it boost your CDR to +50%. If you are worried about running out of mana don't be. by this time your mana pool should be about 700-800 depending on your level and you spells only cost 100 mana each.

You now can complete that one last item, a second Phantom Dancer is just the whipped topping to add to your masterpiece.

Start with Cryptic Gaze, it will save your life at least twice before you get to lvl 3 but you shouldn't touch it again until lvl 14 when you finish it off. I level his Q last because with this build Cryptic Gaze is all about the stun which lasts the same amount of time(1.5 seconds) whether its lvl 1 or lvl 5, you are going to deal plenty of physical damage without needing the increased 50 damage for each lvl.

Max out Enrage ASAP and leave it on AT ALL TIMES, but don't skip your ultimate Cannibalism at levels 6 and 11. Enrage grants you permanent health increase with each creep kill you record while it is engaged(doubled if it is a large minon/monster or champion), not to mention the added attack damage. By lvl 6 you should have racked up between 75-100 additional health points, 175-225 by lvl 11, and at end game you should be around 400-600 additional health!! This is estimated with 180-220 creep score by end game.

Max out Death's Caress next because it does damage to an area of creep to help farm and it can also add to your damage taken as it absorbs up to 300(at max lvl) before it detonates.

Level this every chance you get, this is what makes this Sion build so fun. Cannibalism not only doubles your already massive attack speed but it also doubles you life steal which ALSO heals your allies(for 50% of your damage dealt) within range of you. So as long as you keep attacking, your team will win every fight.