So Sion is a fun champ, can be built AD or Tank with relative success, with a versatile kit able to teamfight and splitpush, both of which can be pretty oppressive. His passive gives him a massive amount of tank stats for farming, so I decided to run him mid, the main lane for farming, scaling champs due to its safety compared to top, as well as avoiding his bad matchups like Camille, Fiora, Irelia etc. After a few ARAM games of Sion, I started to see the appeal of AP sion. Your poke is really hard to dodge if it is placed well, with an insane range, slow, armour shred and even damage with comet. On top of this, Sion W has dual scalings, which can lead to some fun builds on other champs (take AP sejuani). Furthermore, Sion has pretty insane base damage on his q and r already, since he is a tank, so I thought that building AP to buff his e and w would be fairly appropriate. AP sion mid is a strong laner, with a shield to ward off assassins and his e poke being high range and high damage, allowing him to deal with mages. Being able to deal with two main archetypes of characters seen in mid is pretty huge, having a lot of good matchups. Scaling is still very good, because of your W passive. AP sion allows you to sit somewhere inbetween a tank sion and an ad sion. AD sion has large AoE damage with r and q, but not tanky because w is smaller and less resistances, tank has less AoE damage but more tankiness because more armour and bigger w. AP has the AoE from R and W, with night harvester proc while also having respectable tank stats due to large w, more hp and tanky itemisation later. On top of this, your pick potential in side lanes is pretty huge, with your massive e range and slow being able to prevent people from escaping and allowing your team to collapse.

You should try and join every fight possible. Your waveclear isn't the best early because you aren't maxing q, try and hit your e on the casters and the enemy at the same time. Your R allows for fast rotations, and note that you should cast w mid r. If you hit the R, you can quickly burst a target with W E (W cast is instant). If you can hit 3 or more targets with your w and night harvester while ccing a few other targets you should win that fight.

The reason you don't build full AP sion is that Sion's AP ratios are pretty lacking, so rabadons wouldn't give much value. You can compare the damage between buidling an abyssal and builing a rabadon's 3rd and I imagine the damage difference isn't too big. This build revolves around Sion's high base damage on his abilities and amplifying it with flat damage boosts, i.e. night harvester dealing 250 damage base (15 ap ratio doesn't warrent building too much more ap), then you have damage multipliers in Horizon and Abyssal allowing you to amplify your existing high damage rather than trying to scale a low ratio ability. Demonic gives you more tankiness, shield bash + demonic will give you a free 20 armour and mr for pressing w and hitting 1 ability, as well as having the burn to make your AoE damage sting harder, along with the synergy between demonic and abyssal + horizon making the burn deal more damage.

In lane, build around the principle that flat damage is key. Rush an alternator and proc it on enemy champs as much as possible whenever it is off cooldown. Trade around your comet and manaflow early, aiming to stack it as soon as possible. You don't have too bad mana issues if you do this, so a tear is unneccessary as well as seraph's being useless as an item. Aim to force them out of lane with your poke, if this is not possible allow them to shove a wave to you, ask your jungler to stay near and aim for an e, if it hits they will be slowed and unable to run. Use ult to chase if neccessary.

Midgame, group with your team if possible, but try and take as much cs as possible from side lanes. You can survive better than your teammates, so give them the relative safety of midlane. Use your ult for fast rotations if anything happens. You will be the primary engage most games, so do that, but you also don't mind sitting back in more poke based comps either, making you flexible.

Late game you just play like a typical tank Sion with a bit of bonus damage from your NH on your engage. If you die, use passive to proc night harvester on any targets you haven't managed to hit.