Welcome to my guide to AD Pseudo-tank Sion, with superstrong lifesteal. Are you ready to 3-hit their carries and walk out at full health?

Pros:
+Easy to farm with such high AD
+Ult has fairly low cooldown
+Extremely strong teamfight once farmed and has items

Cons:
-Has extremely high mana costs early on
-Can be outsustained by some champions
-If focused in a teamfight, can be taken down quickly (feign weakness and it shouldn't be a problem)

Ignite can be used effectively, especially when trading with people who can sustain well (like Aatrox, Sion, Nasus, Dr. Mundo...) but I don't feel as though it's very strong on AD Sion, as such I rather Exhaust and Flash. Exhaust makes it easy to take out single targets in the early/mid game and to focus and stop the enemy ADC lategame. Flash is obvious. It's an escape tool on a champion with no movement abilities and can also be used offensively to take down opponents quickly. It can also be traded out for Ghost, which can be used to land more autoattacks on some champions.

Feel No Pain/Passive: Basically, every now and again autoattacks against Sion deal less damage. Very strong early game.
Cryptic Gaze/(Q): A powerful ranged, single-target stun. It can guarantee you or your team a kill, or help you get away from ganks/chases.

Death's Caress/(W): A decent shield that if not dealt with can explode and deal AoE damage. Can help with last hitting/tanking minions early game and can shield some damage during teamfights.

Enrage/(E): A must-have on Sion, once it has a point in it, don't turn it off. Constantly stacking health will make you tanky in lane and unkillable in the lategame. A good farmer can get 3,000+ at level 18.

Cannibalism/(R): Maybe one of the strongest abilities in the game. With increased attack speed and lifesteal, you can easily outsustain anyone in a straight fight. If you get low and have to back out of a fight, find some minions or jungle creeps to lifesteal from then walk back into the fight. For example, I walked out of a fight being chased by enemy Rengar on only 300/3,000 health. Walk to the Wight camp, lifesteal a bit, and turn around on 1500 health. Win the fight, and re-enter the teamfight.

For safety you should start with your stun first, to prevent yourself dying to strong level 1 enemies or to invades. Then get your E to start stacking your health and W to prevent harass or to help farm. Then prioritise maxing E, then W, then Q, while getting points in your R whenever possible.

Obviously, you would pick your starting items based on what enemy you are laning with. Most of the time, a Dorans blade will help you enough, but get a Crystalline Flask against lanes like against a Singed, or Cloth armor against superstrong AD champions like Riven, Fiora, or (you never know) another AD Sion.

Boots are definitely required until lategame. With at least an additional 10% movement speed from items, boots would not be necessary with your stun on a decent cooldown.
Ravenous Hydra can be used to greatly improve your teamfight if you are already doing enough damage to single targets, and also provides a large amount of lifesteal.
Any of the situational items can be used to replace items in the usual build. Be careful with what you're doing however. This build requires that Sion has an enormous amount of attack damage, lifesteal, movespeed, critical strike chance, and attack speed. If any of those components is missing, his strength will be greatly lowered.