The Soul Furnace buff displays how much shield strength is remaining, use this information to time its explosion perfectly.
Playing Against Sion
Even if Sion still hits a Decimating Smash, making him release the charge earlier reduces its impact.
Use the time after Sion has died to reposition and prepare for his return.
Sion relies heavily on good skill timing and placement in order to be useful - the majority of his kit is unreliable in terms of hitting abilities, but they synergize well with each other to result in incredible sustained damage.
His passive, Glory in Death, is useful for revenge kills but is most prominent in teamfights where enemies will be under threat by the extra DPS he'll have while reanimated.
Do not underestimate the damage he deals with Glory in Death. With each hit dealing an additional 10% of the target's max HP, Sion can decimate squishy/slow targets.
If you have purchased a Titanic Hydra the attack reset of the active combined with the bonus damage can provide a large amount of unexpected burst.
Decimating Smash deals AoE damage and when well placed can knock up multiple enemies. Fully charged and well placed, this ability can singlehandedly turn teamfights. While the potential threat of Decimating Smash is huge, getting it to apply the brief slow can be enough to pick off a target.
To more reliably hit opponents, try to hinder the their movement before charging; having your teammates apply stuns or slows beforehand or hitting first with Roar of the Slayer or Unstoppable Onslaught will make this considerably easier.
It is important to note both when playing as and against Sion that his Decimating Smash is NOT visible if he is not visible as he charges it. If you can hit your enemy over a wall, around a corner, or from a bush, this is an effective way to surprise enemies.
Alternatively, Decimating Smash can be an effective tool to zone opponents. For example, charging it on an area to cut off an enemy's escape route or protect an endangered ally can be well worth it due to its decently low cooldown.
A fast cancellation of Decimating Smash will still deal one third of the maximum damage and slow the opponent briefly.
Soul Furnace makes Sion very durable with both components, as he can gain quite a bit of health from the passive and a large shield that scales off his max health to compliment it.
Though the shield may not persist for very long in early teamfights, it is still a powerful damage tool due to its AoE % max health damage.
Soul Furnace can be cast while using Sion other abilities. Casting Soul Furnace preemptively may allow you to detonate the shield earlier by avoiding the delay.
Roar of the Slayer grants Sion a ranged AoE slow with boosted damage or with armor shred and as such is effective tool for initiation.
Maxing Roar of the Slayer first while pushing minions in lane grants Sion serious long ranged damage poke and wave clear.
Since the damage is magic, it is effective against early armor stackers.
Though the armor reduction only applies to the main target, it is a reduction and thus will make any enemy that much more vulnerable to physical damage bursts from you and your teammates.
Unstoppable Onslaught allows him to move a long distance while also dealing high AoE damage and providing a long knockup.
Canceling Unstoppable Onslaught briefly before hitting the intended target will let you stop and deal damage at that position - even if the opponent gets away. This way Sion will hopefully be placed right where he belongs - in the middle of the fight.
Taking Manaflow Band can help with your early game mana problems.
Sion loves to poke a lot in lane, so you can easily build up mana.
Many tank and fighter items work well with Sion, but generally some good options support the hard-to-kill, sustained threat that he can be.
Movement speed will improve Sion's potential to engage a fight without his ultimate and slows will make it easier to land his abilities. Both make chasing and Glory in Death even more threatening.
Boots of Swiftness can be incredibly effective against teams lacking in hard CC but with lots of slows.
Righteous Glory can be an excellent item for Sion, giving him some extra durability and an active AoE speed up and slow to boot. It can even be cast during Unstoppable Onslaught, meaning a well timed activation can allow your team to follow up and collapse on the enemy quickly.
Note that it may be better to opt for other defensive items, as Sion already benefits from theoretically infinite health scaling and already has several tools to engage.
Dead Man's Plate could be a considerable alternative for Righteous Glory if Sion needs more mobility for himself, tank and damage mainly coming from his Unstoppable Onslaught. As Sion is using Unstoppable Onslaught, he builds up stacks for Dead Man's Plate to make more damage for his next auto attack when hitting somebody but if he hits a terrain or wall, the stacks of Dead Man's Plate still apply as he's waiting to hit someone with his next auto attack which will still do the same damage unless Sion is stacking it after he used his ultimate.
Trinity Force grants the most movement speed possible from a single item along with an abundance of stats that Sion can use, but is primarily offensive and is still expensive.
Iceborn Gauntlet is a decent item as it provides mana and cooldown reduction, while also granting Sion armor and a slow field, making it easier to chase down opponents, though it lacks the higher defensive stats of Frozen Heart.
Ravenous Hydra can be a decent item, adding to his waveclear, granting him more sustain, and some more AoE damage in teamfights, though is purely offensive.
Alternatively, Titanic Hydra is more defensive but gives similar AoE damage that scales off max health gained from Soul Furnace.
Another strong offensive pick is the Maw of Malmortius. Hexdrinker will make for a good defensive tool against mages early, granting resistances and a shield, and once upgraded, will make him more of a threat at low health, especially when considering his passive.
A Liandry's Torment is decent if you have to deal with a fair amount of tanks and are frequently using Roar of the Slayer. With all the CC and AoE damage in his kit, he can make good use of the burn passive along with a slight boost to his shield.
Youmuu's Ghostblade can be a decent item for snowballing a lane in your favor as Sion has respectable base damage on his abilities. It also possesses an active that allows you to chase well, both alive and reanimated.
Blade of the Ruined King is a decent niche item for dealing with champions that can interrupt and kite fairly easily. While decent as dueling item, he does not benefit from the attack speed as much since his attack speed is fixed during Glory in Death.
Sion gets tons of HP with the passive of Soul Furnace and has a shield that scales with maximum health, making defensive items exceptional on him. Though health is fairly appealing, it is often better to get armor and magic resist to compliment the extra health unless the enemy has an unusually large amount of true damage.
Guardian Angel can be a powerful defensive item, giving him a significant amount of armor and magic resist and allowing him to be even more of a threat with another revive.
As enemy teams may ignore you because of this item, Guardian Angel is effective on offensive Fighter builds. In tandem with a Maw, Sion will become a threat that is difficult to bring down but can't leave along.
Warmog's Armor carries a large amount of health and sustain that scales with max HP. Its passive will force enemy teams to commit to killing Sion or risk him returning fully healed, either situation being unfavorable for them.
Frozen Heart is probably the best choice for armor, because the high amount of cooldown reduction supports Sion's sustained ability damage and CC. Furthermore, the attack speed slow works well just by staying in the middle of the enemy team. The lack of health on Frozen Heart hurts Sion less than most other tanks.
The build path via Glacial Shroud is quite helpful early against an AD lane.
Sunfire Cape is a good item as it provides favorable defensive stats along with even more sustained damage as you'll often be in the middle of a fight regardless.
Randuin's Omen, while a staple on nearly every tank or bruiser, synergizes very well with Sion, granting him yet another AoE slow along with a high amount of defensive stats.
As Sion's strength lies in his amazing late game with infinite HP scaling, playing an early game champion like Renekton, Riven, or Garen can hinder him from reaching his full potential.
In short, anyone who can interrupt his Decimating Smash and burn through his Soul Furnace shield quickly and early will be able to push him out of lane as well as deny his usefulness during teamfights.
Soul Furnace's shield can deal a large amount of damage to your team if you don't break through it fast enough. If you cannot destroy it within the three second period before it can be detonated, consider disengaging and letting it expire, as it has a fairly significant cooldown.
Pay attention to the game's audio as you can hear the noise Sion'sUnstoppable Onslaught makes globally, and position yourself to dodge out of the way.
While the engage potential of Unstoppable Onslaught is significant, the cooldown is very high at early levels. Keep this in mind if he uses it defensively.
Without Unstoppable Onslaught, Sion is very vulnerable to kiting. Try to engage or force an objective while it is down.
Sion does a large amount of mixed damage, especially in the laning plase. Avoiding his abilities early is important in securing the early game against him.
Roar of the Slayer is relatively fast compared to the rest of his skills, but its high base damage and slow makes it important to dodge to avoid follow up.
The empowered damage only applies if you are hit by a minion or monster. Try not to give Sion the opportunity to hit you with one.
Sion excels at zoning low mobility opponents with his abilities. Ryze and Teemo can be effective against him, provided they do not get too close.