Welcome to a short support Sion guide.
I will be your guide for today.
My name is TrapsAreNôtGay and I'm a Trundle top/Sion Support Main
Let's get straight into the Guide

Aftershock: You wanna have aftershock because 3/5 of your abilities are CC and Grasps of the undying won't be too useful as you will only use W+Q+E or W+E+Q combo and not be in melee range most of the time

Font of life: I simply chose to take sustain instead of pushing power but demolish can also be used instead

Condition: It's really great for armor and MR as the % scales to late game while the base is decent for early game

Overgrowth: This is a must have for Sion. His Passive and W scales with Health and you will get farm by using E and Relic shield

Triumph: It's great for extra gold in the early game, while the healing is great for teamfight because you will most likely take all the damage.

Legend - Tenacity: Sion does not have great mobility which is why I take Tenacity so the CC won't last as long

Okay, you're probably thinking right now:
"Warmog's as number 2 Item?"
Okay, so you have to understand how well Warmog's works with overgrowth and conditition, as you don't have to worry about really resiet and can build more armor so you can stay in lane longer

Frozen heart: Great for taking AD damage in Early-Middle laning phase.

Spirit Visage: Great with Warmog's and also if the enemy has a AP support like Brand/Lux/Lulu

Gargoyle stoneplate: Okay So for the core build I had to chose between Gargoyles and Thornmail and ended up with Gargoyle, becuase in general it's gonna be better at any point in the game, but if you have 3 or more Auto attack reliable champions you should really go for the Thornmail instead.

Ignite: The Only reason I take Ignite as my primary summoner spell is because of the 60% damage you do solo at level 1 with a sinlge combo: Full Charge Q + Auto attack + Ignite.

Flash: Cause of obvious reasons

Teleport: Teleport can also be used but it's only if you expect to roam alot, if you plan on staying in lane more than 80% of the time you should stick with Ignite, Smite or Exhaust

Smite: The reason you want smite sometimes is because of the passive from Cinderhulk which can make you EXTRA tanky. (also extra true damage when taking Herald/Drake/Baron

Exhaust: Only take this if you're vs a Vayne or Kalista.

Red side: You will go to the lowest entrance to the river and stand there until 1:25. When the time hits 1:25 go to the Blue side bottom tri-bush and wait for the enemy carry to approch it. When you see them, begin to charge your Q and have Ignite ready. If you land the Q you should engage, but if you miss you need to run ASAP. (In my games Red side has a 100% chance of giving first blood with this trick)

Blue side: You will go to bottom tri-bush and wait until the time hits 1:28. After 1:28 go to the bottom river bush and wait for the enemy carry to approch. Same thing as with Red side, Charge Q, Have ignite ready, and kill them, of cause if you miss the Q you will have too run again. (Blue side only has a 86% first blood chance when using this tirck)

Cons

- No Leap/Dash/Teleport/Jump
- Weak middle game
- Requires alot of bush camping to work early game
- High mana cost and low mana pool
- Takes a long time to get used too

Pros

- Strong Early and Late game
- Good scalings and Base damage
- Tons of CC
- Does not require items to become a beast
- Armor shredding E
- Broken hixboxes (Q has around 30 more units in range than animation shows) (W has around 70 more units of range than show in animation) (E has around 50 units more range than animation shows) (R has around 200 more units of champion collision range than shown in animation)
- Can go AD for a fun time
- Has syngery with every ADC