This is my Dominion Build for an AD-AP hyrid, critting, dodging Sion.

It's a bit unorthodox as far a hybrid build, as it focuses more as AD than AP.

First real build on here, so comments and suggestions would be great.

Any questions, feel free to comment and ask.

For Runes I use

(9) Armor Pen Reds


(9) Crit Chance Yellows

(9) Cooldown per level Blues

(2) Armor Pen Quints
(1) Crit Chance Quint

I used to have 1 Magic Pen Red and 1 Flat AP Quint, but those fell out late game.
Instead I put the last Red into Armor Pen, and the left over Quint into Crit chance.

For masteries, I go a bit off for a 21/9/0 build.

Offense:
I take 3 in Archmage's Savvy (AP boost), 3 in Deadliness(Crit Chance boost).
4 in Alacrity(Attack Speed).
3 in Sunder(Armor Pen), 1 in Offensive Mastery(Minion Damage).
3 in Brute Force(Attack Damage).
3 in Lethality(Crit Strike Damage).
1 in Havoc(Increased Damage).

Defense:
I take 3 in Resistance(Magic Resist), 2 in Hardiness(Armor).
4 in Evasion(Dodge chance).

The reason I don't use Utility masteries is because Sion is rather squishy in this build. I don't get many armor or magic resist items, and you don't really need the upgraded Ghost. Also the dead timer is a lot smaller, so that mastery isn't helpful.

As far as Items, I mainly stick to this build. Although Sion is great when going straight AP, a hybrid version gives him full control over any situation.
Say you go all AP, the enemy gets some magic resistance, and you do nothing now.
Or you go all AD, the enemy gets some armor, and you have no where to go.

I usually stick to the same build each game, because it gives about 120 magic resist, 180 armor, and 4k health. It's well balanced and can help you survive most fights.
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To start, get a Prospector's Ring for the early game AP lane control, and Boots of Speed for now.
Health or Mana pots are up to you, depending on how you feel.
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Next I upgrade the Boots of Speed to Ninja Tabi.
Ninja Tabi gives great dodge chance, especially since you have points in the mastery. Also the armor it gives will help you stay alive longer.
I don't suggest getting Mercury Treads. The tenacity isn't worth it if you pick up Cleanse, and the magic resist you can get from an Abyssal Scepter.
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About now, you should have at least 1 kill, and possibly putting some heat on the enemy bottom tower.
Depending on how the game is going, I either get a Negatron Cloak (if there is alot of magic damage being thrown around) or a Blasting Wand (if you're doing well as far as kills).
I upgrade those to an Abyssal Scepter which gives some defense against magic damage, and good AP boost.
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People are gonna hate you for this next part.

ATMOGS

I start with a Giant's Belt if you have the money, or a Red Crystal if you don't. Upgrade this to a Warmogs.
Warmogs gives you a super amount of health, and it helps your squishy-ness
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After I finish Warmogs, I get a Chain Vest, and upgrade that to an Atma's Impailer.
Atma's gives you crit and damage per percentage of your max health.

With the crit yellows and the all the armor pen you have, critting is amazing, and you'll be doing 200+ damage with regular attacks.
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The last item you get, is completely up to you... I choose Phantom Dancer as my final item. Attack Speed, Movement Speed, etc. Such a good item.

I suggest something with health so your Atma's does even more damage.
On the flip side, you could get more armor or magic resist if you feel you are dying too fast, or you could get more damage.
Possibilities are Entrophy, Frozen Heart, Black Cleaver, whatever you feel will help you and your team.
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Guinsoo's Rageblade

Although this build is focused more on AD than AP, Rageblade isn't completely out of the question. It's not a bad idea if you want more of a balanced hybrid, and as a late game item, it puts a good amount of pressure on the enemy team to stay away from you.
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Ionic Spark

Another good AD item, especially since it gives sort of a splash damage effect. Great early game instead of getting abyssal scepter. Also helps with minion kills, which gives you more long term health.

For Summoner Spells I use Ghost and Promote. Ghost is a lot better than flash for Sion in Dominion, but Promote is interchangeable depending on the other team.

If there is a lot of Crowd Control (stuns, taunts, fears), I swap Promote for Cleanse.

The main reason I use Promote is because this build is a little expensive, and it gives you a great amount of extra gold.

Pros ~Early game dominance, late game domination.
AP gives good shield and stun, AD gives good damage and life-steal (with ult) and can take out a nice chunk of someone's health. Cooldowns and mana pool aren't that big of a problem, because you're more focused on AD.

Biggest Pro of this build? Sion has 4k health! With Atma's, thats a ton more damage, and you become a lot less squishy than most other characters.

Cons ~Focused very often. Kinda squishy until you get Abyssal and Warmogs. A bit slow as far as regularly roaming. Weak against AD carries.

"Laning" and Gameplay:
I take bottom tower by myself, and hold as long as I can.

More than likely you will be going against a Heimerdinger or some kind of carry. Try to use Promote and your Enrage to gather some minions for the extra health. Have 3 of your fastest teammates sprint to Windmill. Let someone else take Middle Tower, then help your team at Windmill.

Sion does not benefit all that well from the Center Buff, but if you want, go for it.

Ghosting puts you at about 435 movement speed, which isn't all that bad for roaming or trying to catch up with someone.
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DO NOT TRY TO TOWER DIVE WITH SHIELD ON.

It's a bad idea. The tower will take it out in 1 or 2 hits, and then you're subject to heavy cc and massive damage.

Try to have someone capture the tower, as you stun the enemy and put on some damage.
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Do not try to 1v1 Jax, Wukong, Garen, or any pure DPS champions.

Lee Sin, Akali, Singed, and Heimerdinger are easy 1v1 targets. Most tanks don't put out much damage, so if you put on your Death's Caress and turn on your ult, you can out DPS and kill them. Also you can take out AD carries like they are nothing.

Shield, Ghost, Stun, pop Shield, auto attacks, ult = Massive damage.
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Even though everyone usually has mana problems with Sion, this build focuses more on straight damage. You should only be using mana to put out stuns, and activating your shield. Neither cost more than 200 mana, so your small mana pool is not as big of a deal.
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Well that's my Sion build for Dominion.

Please like, comment, suggest anything that could help the build as far as information or plain aesthetics.

Have fun being OP =]