There aren't many Champions as flexible as Sion. Currently, most people build him with AP and take him Mid or maybe Top. Although Sion hasn't been changed recently, the new item Feral Flare is a great addition to his arsenal. Feral Flare overrides the damage Sion takes from his Enrage. Combined with some attack speed runes and the mastery Vampirism, Sion can sustain in the jungle for a fair amount of time. Though Feral Flare is quite effective on him, he is not built like most champions who use it. Jungle Sion is best built as a very tanky bruiser. Without further delay, let's delve into a more in-depth look at Jungle Sion.

Early Jungle:

Like all junglers, it's important to start Hunter's Machete, 5 Health Potions, and your choice of trinket. I prefer to start blue buff on purple side, and red buff on blue side. This start is effective and efficient as an early top gank happens all too often. Farming creeps, however, is your priority; building stacks on Feral Flare takes a little while, and it's best to get it as early as possible. It is also important to note that you don't have to kill a monster (or minion) with Enrage to get the health boost. It's tpyically best not to recall until you can afford both Madred's and Avarice Blade, if you need to recall earlier due to health, that's perfectly fine, the important thing is Avarice Blade (side note, always max your Enrage first. The max health increase scales into late game like no other).

Avarice Time:

Now that every monster you kill gives you health and gold, it's time to really grind out those stacks. Avarice shouldn't be built into anything for a very, very long time. While getting stacks, it is critical to pay attention to the lanes. Sion is pretty bad at ganking, yet his counter ganking skills are superb. His ult heals nearby allies and his stun can prevent an over extended enemy from escaping their erroneous ways. Don't worry about blowing your ult, as the cooldown isn't crippling, it also makes soloing dragon a breeze. After you've saved some lanes and farmed some creeps with manly flair, it would be best to recall and finish Wriggle's Lantern. With any gold you have left over, start building into Spirit Visage.

Mid Game:

Now you get to the mid game, this is where you should be getting the last few stacks of Feral Flare (unless you haven't had to gank). After Feral Flare is finished you are now ready to team fight and/or build more onto Feral Flare. With all this, killing almost anything means you gain a long list of buffs: Damage, Health Restore, Health, Bonus Gold, Testosterone. By now you should have around 550 bonus health from your Enrage +/- 100. You can now deal damage, and take it, and heal the damage you took. Sion serves best as a frontline tank in team fights. He is impossible to ignore as he heals himself and all his nearby allies. Even with Ignite on him, Sion still manages to heal a large amount of the damage he takes, the added effect of his passive also makes him even harder to kill. After your first team fight, or maybe even second, you should be able to recall once more. Finish Spirit Visage, then start building an Infinity Edge.

Late-Mid Game:

With any free time you have, farm the jungle as much as possible. It is imperative to help your Enrage push your tankiness even further. Most of this time is spent doing the same thing as Mid Game, unless either team sieges up the other's base. Sion doesn't do much to help siege an enemy base, besides eating skill shots and dissuading an enemy engage with his stun. On the defensive, however, Sion can be a dangerous foe, as his ult can heal any damage he and his allies took from poke. Beyond on that, there is not much Sion can do besides protect his team and build his gold stores up. After you've been given a brake once more, recall and finish Infinity Edge. After that, you should be able to out right buy or almost buy a Warmog's Armor. This should be able to let you finish the game for your team, or prevent the enemy from doing so.

Late Game:

This is where Sion becomes an unstoppable force, late game is where nobody on the enemy team wants you to get to. Sion at this point is a juggernaut that can hold off a portion of the enemy team by himself for a while, and though you may think it, Sion can't 1v5 unless the whole enemy team is vastly behind you. Farming is helpful, but you shouldn't have much time to do so, soloing dragon and gearing up for baron are of the upmost importance for Sion right now. However, when you have the chance, recall and buy Atma's Impaler, Sion is probably the only champion you will ever see buying this item. With enough stacks on your Enrage, Atma's can give you anywhere from 70 to 100 Attack Damage. along with crit chance to work with your infinity edge, and armor to make you even tankier. At this point, the game will finish at any time, and you will either win, or you will lose.

Preferred Final Build:

Any Boots, Wriggle's Lantern, Spirit Visage, Infinity Edge, Warmog's Armor, Atma's Impaler

This is what I usually build on Sion, however Sion is extremely flexible. You can build any item on him and it will be useful somehow. Dropping the Infinity Edge for Blood Thirster is a good choice, and trading Atma's for something like Randuin's is also a great decision. It all depends on what you like, and what the enemy team would not like you to build.

Conclusion:

Jungle Sion can be an effective tank if your team doesn't have one, or you are looking to further your team's overall tankiness. Sion works best with people who can jump in and make full use of his ultimate. Besides knowing when to go in, and when to avoid team fights, playing Sion is almost as easy as Garen. And remember, no matter how well you play, you won't succeed if you don't work with your team. Find a balance between farming, ganking, and team fighting. There isn't much else to it, get out there and have fun.

Other great Sion guides: Conan the Barbarian, Terminator 2, True Lies, Predator, Commando, Jingle All the Way