I just want to start this off by saying that I am no genius nor the best Sion player ever not do I think I am, meaning that some of if not all of this guide will be flawed. My word is not law, and I implore you to mess around with Sion jungle builds, syngeries, and matchups yourself. On top of that, everything I have written in this guide is subject to change based on my own findings or suggestions from the comments (hint, hint.)

Why Sion Jungle?

While probably not the best way to be introduced to the champion as a first-timer, I find playing Sion in the jungle to be surprisingly fun for how uncommon it is. (I, myself, have only ever played with or against Sion jungle two-three times, and my main account is level 289+ with one other level 27+ account, if that gives you an idea of how much I have played.)
At the moment, I don't play ranked, and when I have, I have played top lane or support--not Sion jungle, meaning that I cannot give you personal confirmation that this will indeed get you to high elo or whatever. I'm frankly scared of both ranked and playing Sion jungle in ranked. It's off-meta, and probably not good on its own unlike whatever champion(s) has a 54%+ winrate at the time of you reading this. It's one of those things such as jungle Taric that require you to be good at both the champion and the champion in the role in order to reliably do well.
I'd say that jungle Sion is worth a few matches to see if you'd like it. I was miserable my first match as it, and in my second one, my Caitlyn took my red buff with her Q at level 1 because I was too depressed to smite the buff when I saw her channeling her Q. Still in that match, even with that happening, I ended up doing well, giving me the drive to try some more jungle Sion, which I later binged for a few weeks!

By the way, in my descriptions under matchups and builds, I occasionally used "//". This was to represent an space between the preceeding and succeeding text since space between lines of text do not show up in those descriptions.

Timothy_Fly's links: I am not this guy, but he is not only a fellow Sion jungle enthusiast, but one that live streams.
Twitch
op.gg

Sion can run a large variety of runes. To make it easier for you to determine the value of different runes on him, I have created a personal rating below of every rune.



I believe that, to Sion, each rune is like a Lego piece that can be pieced together to your liking due to how much he can run well or at least decently. I like generally taking the Percision tree because it always feels at least pretty good, in my opinion (hence why I put the Percision tree as the secondary tree in my example pages); however, I would recommend trying out different combinations in different scenarios to find what you enjoy playing the most.

Grasp of the Undying vs Aftershock

This is mostly here just to let you know that I plan on comparing the two eventually once I test Aftershock more with my current itemisation. Here are some obvious things to get out of the way:

Grasp of the Undying:

Pros:

• Can be activated multiple times in a single combat encounter.
• Heals a tad bit of health (about half of the rune's pre-mitigation damage).
• The combat timer starts off monsters, minions, and receiving damage.
• Grants a tiny amount of bonus health with each proc.

Cons:

• The first proc requires you to be in combat for four seconds, which is kind of a long time in this game.
• Single-target damage.

Aftershock:

Pros:

• Once you get some bonus health, does much more damage than Grasp.
• AOE damage.

Cons:

• Requires you to land a knock-up.
• Can only (realistically) be procced once per combat encounter.

Okay, so, after testing Grasp of the Undying and Aftershock, I'd say that while Aftershock does not feel perfect, it feels better than Grasp. While Grasp would theoretically help more with extended fights, the damage brought by Aftershock in addition to it temporarily boosting your resists makes it feel rather nice. Unless you are fighting a season 12 amount of true damage, the resists help you way more than Grasp's heal. Against Olaf or Morgana's black shield, it feels :(, though.

The synergies section was made with the questions in mind of how how well can a champion follow up on Sion, how well can Sion follow up on a champion, and how well do their abilities work in tandem with one another.
In my opinion, you don't always want to be the one engaging as Sion. I believe that he can oftentimes do well, if not better, as immediate follow-up rather than the inital engager. An example of this would be Warwick ulting a carry followed up by Sion ulting and/or Q-ing the suppressed carry. Generally, I find Sion best with champions with high CC and/or consistent/low cooldown damage to backup Sion's own damage on those he cannot burst(assuming you are building damage.)
This section of my guide is the worst in my opinion due to how many champions I tried to include in its listing with how differently you could play Sion. I apologise for how rough it is; though, I would still hear out any of the listings for a general idea. All of the matchups with descriptions provided are the ones I am most confident in my rating of.

Below are two videos of me performing full clears. I used the "refresh cooldowns" button to refund a smite after using a smite to simulate recieving a leash. You do not have to put control wards in the places I put them in the videos. Those were just examples of helpful locations.
Note: Starting raptors with Q works, but I personally prefer starting on a buff with W.

• Press W at 1:27 so that you can get your first W detonation off sooner.
• It is more helpful to detonate your W whilst clearing than to let the shield get destroyed. The aoe damage it provides is helpful, particularly so against camps with multiple monsters (raptors).
• Try to avoid resetting your autoattack timer by detonating your W shield mid-attack. Either detonate it yourself early or try to kite the buff for time.
• When starting red, make sure not to detonate your W too early or else the baby raptors will not die to your jungle item tick damage.
• Do not waste time finishing off a monster (other than the Red Brambleback and Blue Sentinel) that is at ~30 health or so since your jungle item tick damage will kill it off.

Play Video Play Video

Lord Domonik's Regards vs Serylda's Grudge

These are the only two options Sion has for instant %pen, and neither of them look spectacular on him on paper. I would originally build Serylda's as I didn't want the crit and Lord Dom's passive is useless on Sion. In practise, Serylda's passive isn't great either, as your only ability that does not already slow is his W, which is still usable. After seeing other Sions always take Lord Dom's over Serylda's, I gave it a go and I do prefer Lord Dom's, yet I do still think Serylda's is usable. The greater %pen of Mr. Dominant's Regurts is the best reason to take it, with the crit being secondary.
tldr: Take Lord Dominik's Regards, but try Serylda's Grudge if you are curious.

Why no Black Cleaver, Mr. Guide Man?

To start off, I've really wanted this item to work on Sion. I have tried quite a bit both in matches and in the practise tool to get this item to work on him. It is not viable for a few reasons*:

• It does not provide immediate %pen (Bad for assassinating)
• No armour penetration from the buildpath
• Only stacks off of physical damage (Only Q, R, and auto attacks--none of which are "spammable" to swiftly stack the passive such as Urgot or Pantheon)
• Sion lacks a great attack speed to make up for the lack of spammable, physical damage-dealing abilities

*If you want to be that guy, it could be "usable" into super tanky teams, likely in tandem with Lord Dominik's Regards.