PROS + Can soak up massive damage + Can provide pressure even after death + Great waveclear with his Q + High roaming and engage with R + Great zoning with Q + Has infinite scaling + Rarely banned

CONS - Has very little mobility outside his R - Many bad matchups - Weak to damage based on max health - Q is easily disruptable by CC - Weak if behind - High cooldowns early game - Has mana problems in extended fights

Spells

Teleport: Splitpushing and wave control advantage.
Flash: Outplay and escape potential.

Runes

Phase Rush: For sticking power and chase. Easily activated with Glory in Death. This rune is a game changer for Sion. In my opinion, it far outranks any other keystone for this champion. None other even comes close. Not only does it make a lot of bad matchups against Sion easier, it overall makes Sion a much more fun and deadly champion to play.
Nimbus Cloak: Extra engage tool and escape. It can definitely catch opponents off guard.
Transcendence: Extra ability haste and cooldown reduction on takedowns. Best rune in the column.
Gathering Storm: Helps with scaling late game. HP is one of the least powerful scaling stats in the game unfortunately. This rune makes Sion a bit easier to utilize in the endgame.
Conditioning: Best rune in the column for Sion. The value in gold is there and it makes him alot more resistant without having to build too many resistance items.
Revitalize: Works perfectly with Sion's Soul Furnace and healing during Glory in Death.

Starting Item

Corrupting Potion: Mana and health regeneration for easier laning phase.

Mythic

Heartsteel: This item can really rack up health and damage. Most of Sion's items and abilities scale with health, making this mythic the best choice. In addition to extra HP, the size increase also helps in hitting Sion's Decimating Smash when you build more legendary items.

Boots

Boots of Swiftness: Best boots for sticking and aggression. These boots are a MUST on Sion if you want to make the most of Glory in Death.

Legendary Items

Sunfire Aegis: This item does so much damage on Sion because of how it scales with HP. Not only can it dish out massive amounts of damage in extended teamfights, but it also makes waveclearing alot easier.
Spirit Visage: 25% shielding bonus is a massive buff to Sion's Soul Furnace and goes amazingly with the healing during Glory in Death. Paired with Revitalize and the Chemtech Drake buff, Sion can rack up absolutely ridiculous amounts of shielding with Soul Furnace.
Titanic Hydra: Waveclear faster and scaling damage infinitely with health. Best item for an AD damage spike.
The Collector: This item is a nice ending item. Sion can absolutely delete squishy targets after building this item. The crit also comes in handy and the execute is always a nice finishing touch.

Buffs

Elixir of Wrath: In my opinion this elixir is the best the late game. Titanic Hydra and The Collector synergized with this buff make Sion's Decimating Smash and Glory in Death much more heavy hitting.

Elixir of Iron: The second choice out of the three elixirs, I'd recommend only using this buff if the enemy team has a ton of hard CC. Health isn't the best scaling stat in the game as max health damage is so prevalent. The tenacity is really the only selling point of this elixir. Nine times out of ten, go with Elixir of Wrath.

Knowing When To Die

Sion has one of the most powerful passives in the game if you know how to use it right. It allows you to put pressure even after you die and catch up on XP, gold and kills. If you know that you're in an unescapable position in lane, set yourself up for success when you die. Choose your target. Usually, the closest enemy or enemy that has the least amount of health or no escape option is the best target. Before you die, use your E to slow them down so its easier for you to chase. If killing an enemy is too out of reach, focus on farming your wave so it is in a good position for when you get back to lane or taking a turret. Its always a good idea to think before going into a risky position, don't die in vain. This prevents you from inting and will overall make you a better Sion player.

Using Bushes

If you're in a bush, Sion's Decimating Smash can't be seen by enemies outside of it, given that the bush is unwarded. Use this to your advantage to catch enemies off guard. and start off with a solid stun.

Timing Your Q

Knowing when to release your Q is a great way to get better at Sion. Decimating Smash is the bread and butter of Sion's kit, and knowing it well will take you far. Play mind games with your opponents. If they are about to leave your Q, hitbox, release earlier to still do damage and possibly get a small stun. After a while, mix it up and fully charge your Q even if they walk out. I've definitely used this to my advantage before, as enemies are used to you just releasing it right away after they walk out. Sometimes they'll walk back into it out of habit and you can hit them for the full stun.

Coming Back to Lane

Many players are unaware of the sound your ultimate makes. Use this to your advantage to get back to lane and suprise your opponent while they try and take your tower, thinking you're gone. Start your ulitmate further down the lane and give yourself enough room just in case they do notice your ult sound or try to escape. Stun them with your R,follow up with a fully charged Q and then use your E towards the end to slow them if they're still alive. This is a really nice way to get an easy kill and many players don't expect it.

Patience

Sion has a lot of bad matchups. There are many enemy champions that can destroy Sion early on and make you fall behind if you're not careful. If you're against a bad matchup such as Darius, it's always better to be patient, freeze wave and farm under tower than to int and fall behind. This goes for any champion, but it's especially important on Sion.