Thebausffs

Guide by the greatest Sion player who inspired my item selection for this guide. You can also watch his videos and streams to learn how he plays more extensively.
Below is an essential guide for Sion players, relevant part of the guide is from 10:45 to the end. In the description there is a spreadsheet which explains how to deal with each matchup, although some advice might be outdated. Play Video

Basics

One of the most important things to remember is to have a consistently high CS rate so you can be useful. Try not to miss any CS in laning, proxy, take enemy jungle. Don't be afraid of dying, but still learn from your mistakes so you can die for a better reason next time. Being aggressive is the only way to improve. As long as you keep this in mind, you can play inting Sion in your own way.

Try to predict which lane will be the most annoying for the enemy to deal with you in. Make sure to put wards close to towers if you want to Teleport there later.

Deciding which build to play:

Tank or AD: This will depend on if your team needs a tank or more damage, as well as how much anti-tank the enemies may have. The spreadsheet in the description of Thebausffs' guide linked above shows which build is better for which matchups. If you're unsure, tank is generally a better pick than AD. AD is not in a great place at the moment.

Best ban: Jax

Core (Tank)

Take this every game unless you need a situational item first, in which case it may be better to skip Heartsteel all together, as it's important to start stacking it early. You should always look to get more stacks but it's not worth it to die for just for stacks.

This is not an item you have to change your playstyle around much, aside from auto attacking more, which you already should be doing when you have the chance. If you're always dying before you can even do anything, especially if several enemies target you simultaneously, it may be better to take Gargoyle Stoneplate before Titanic.

Similar to Zhonya's Hourglass, you should wait for enemies to start directing their cooldowns on you before you use the active of this item, to absorb the most damage possible and take their important abilities away.

Pretty straight forward item. Just don't use Gargoyle Stoneplate and probably not Soul Furnace while the Lifeline shield is already up.

Consider spacing out your abilities more to make use of damage over time. This is not a great item, but better than taking a situational item you don't need.

Core (AD)

Essential funny item. Saving the active for during your passive can be wise to get a kill.

Movement speed and lethality choice, can be taken before or after Lord Dominik's Regards. You should build it after if you need anti-tank. Saving the active for during your passive can be wise to get a kill.

You can just build Last Whisper for anti-tank and finish the item as early or late as you want, but you should build this item fully before Infinity Edge.

Can be built before or after Lord Dominik's Regards. You should build it before if you don't need anti-tank.

Once you have 40% crit, buy this. If you don't have space for it because you bought a situational item, you can replace boots or Hullbreaker for it if you'd like, but it's not always the right choice.

Situational (Tank)

You should take this frequently. Make liberal use of aura damage when you can. Aura size increases with champion size so keep it in mind when you increase size due to Heartsteel, Gargoyle Stoneplate or Elixir of Iron. The main decider for taking this item should be if you need Armor, usually when their top in addition to either their jungle or mid does lots of AD, and it's usually logical as a second item.

Hullbreaker is fantastic for Sion, and it lets you take towers, before you have this item you shouldn't bother attacking towers after proccing demolish unless minions will take a plating without you, since you need to be near a plating when it falls to get gold. The minion buff is a great part of this item so make sure to use it. It's usually logical to take this a second item. If you're playing tank and below Diamond 2, you may want to take this every game to solo carry. Otherwise, you should take this item if splitpushing is going well for you and you won't need to teamfight often.

Don't take this too often, but it's good when auto attack heavy enemies are posing a big problem to you. Make sure to use the aura when you can, either for yourself or to help teammates. Warden's Mail is great to take even before mythic and even if you don't plan to evolve it into Frozen Heart, as it provides most defensive capabilities of Frozen Heart for much cheaper.

Similarly to Elixir of Wrath if you're AD, you don't necessarily need to only take this when you have a full build, you can also take it when you need a sudden powerspike to end the game or prevent your team from losing.

If you really need antiheal and this item will work on your target (they have to auto attack you to proc Thornmail). Don't take this too often though.

If their top, mid and jungle are all AP

Vs high CC

Vs lots of auto attackers

Great boots during your passive and for general movement. Take this more frequently than other boots.

Situational (AD)

Take if there's 2 enemies with frequent shields

Take about every game, when it's convenient

If you just need more damage

• If both teams are busy in a team fight, it's your chance to push your lane! But sometimes engaging in the team fight can be a good idea if you have Unstoppable Onslaught or Teleport. It depends on how good the opportunity is, in contrast to the opportunity of possibly taking a tower or two. A good opportunity to do this would be if you've attracted a couple of enemies so you can't destroy the tower anyway, so you can quickly Teleport or ult to the teamfight and easily win it since they are missing members.
  
  
• If an enemy is in your lane and you can't deal with them quickly, consider swapping lanes or grouping with your team.
  
  
• Don't ult if there is an enemy that can create terrain like Trundle, since they can block you.
  
  
• You can press W before Teleporting or while using Unstoppable Onslaught to be able to recast for damage once you get there.
  
  
• Your Q outline is invisible as long as Sion is outside of enemy vision.
  
  
• Use E before Q or R to land them consistently.
  
  
• The end of R can do a bit (a lot if you're AD) of AoE damage to towers.
  
  
• If a tower is being protected, you can proxy behind it. You can also use time between waves to take enemy jungle camps and proxy at their inner towers (as shown in Thebausffs' guide)
  
  
• If you Unstoppable Onslaught at melee range to an enemy, press and hold Q right after pressing R to increase the stun duration and reduce the Ult animation. If you're Ult someone from farther away, recast R before hitting them instead.
  
  
• Unstoppable Onslaught ignores CC.
  
  
• Use Soul Furnace after someone uses their abilities on you, so you can still block some damage but also get the recast damage off.
  
  
• In difficult matchups, contest the wave at very early levels, which will likely lead to your death, in which case you can get some CS and do some damage to the enemy laner in your passive, forcing them to recall when you Teleport back, letting you fix the wave.

RESOLVE

Grasp of the Undying is an excellent rune for inting Sion since it helps you increase your health throughout the game so your powerspike will hit that much harder. Since playing this build usually makes games last much longer, since how you usually win is only by surgically removing their nexus via splitpushing, this will help you profit off the extra time with infinite stacking potential. The extra damage that it deals means you can trade whenever you can use it and usually win! Actually you should have W up to trade, but against tanks you only need grasp to trade. Inting Sion doesn't care much for healing but he doesn't mind it, so Spirit Visage helps you gain extra healing from this rune.

Demolish has always been and always will be what makes inting Sion good. Once you hit the late game, this makes towers melt before you. There isn't really much more to say.

Similar purpose to Grasp of the Undying in the sense that it's abusing the fact that you're making games longer by playing this build. Makes you even tankier and all it takes is waiting 12 minutes.

ALSO abusing long games to become tankier. Just have to make sure you CS a lot which you should be doing anyway. A useful way to CS is to R across the lane, and you'll almost always be oneshotting the minions.

INSPIRATION

Simple way for CSing to be easier despite your tankiness. Using 2 on siege minion s and 1 on a melee minion is a good choice.

Great for movement speed and kill pressure since your E and Q can both proc it.

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