I picked up Sion right after the rework and found him to be really fun and really really tanky, so far he has taken me up to Diamond 3 at my peak. Feel free to message me on why I build him this way.

All of these items are situational and you have no big reason to build them other than they work decently with sion.

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Executioner's Calling: makes any Q you land an AOE of grievous wounds, perfect for vlads or stupid hp regeners.

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Dead Man's Plate: Have places to go, people to slow, strong armor and hp item, honestly though your ult and or Q/E slow work just as well if not better.

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/ Frozen Heart/Iceborn Gauntlet: Frozen CDR items, build if you want 36 second ults and or perma combo slows with a frozen fist.

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Locket(team cant figure out why a poke team is killing them)/ Randuins(stop critting me): AOE abilities either before you ult (give your team a shield) or after you ult (AOE slow them cause reasons).

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Righteous Glory(initiation for dummys): You cannot use this while ulting so proc it right before and enjoy a mini sivir ult. This gives no resists and loses its passive when fully built so build at your own risk.

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Sterak's Gage: If they focus you and you have trouble finding yourself in the teamfight buy this. Gives nice shield and makes you easier to hit/"soak up damage".

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Warmogs: In case you forgot to level W throughout the entire game or need garens passive cause you can't back right now. Unless you already have 300 armor and mr your passive gives a good amount of hp already.

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GA: more armor and mr, If you ever build GA I hope you have at least 6k hp and are backdooring, this is the most efficient way to build salt.

Pros

Gives both Armor and Magic Resist
Increases your already insane health regen
Great if you are terrible for last hitting and/or left lane
Generates gold (shows how much it accumulated at the bottom)
Over 100% gold efficient
Gives movespeed near turrets or portals
If your team refuses to group/ all you want to do is splitpush
Scales with you
Cooldown is the same as how long it is up so as long if it isn't destroyed you can place another before it disappears

Cons

No hp (this is probably the biggest reason not to build it early on)
Costs a good amount of gold
If they have a Nasus you just gave him free stacks
No cdr
They heavily nerfed it so you can't cheese your lane as much.
If you ult into a ZZrot it will alter your direction (counters your ult)

Try your best to only use auto attacks to farm so that you don't push your lane (unless you want to) and to save your abilities for if your lane opponent is going to fight you. Later on in the game Qing the whole wave followed by W will clear the wave instantly.

Your passive, if you die you get a chance to redeem yourself with rapidly decaying hp, one movespeed boost, and 1.75 as autos that deal 10% max percent ad damage to their target. Can be used early to cheese the enemy teams buffs but doesn't give enough exp to get to Lvl 2.

Bread and butter skill, E comboed with Q does a fair bit of damage and is your main combo vs laners.

Sion's identity, last hitting minions and creeps give you permanent health to offset your terrible health scaling. Active is also a shield that scales with your max hp and after 3 seconds can pop doing percent max hp magic damage in an aoe around you. Either use it for damage soaking or to pop it(preemptive charge).

Effects a number of minions (Annie Tibbers, Zyra plants, Jungle creeps, Heimerdinger Turrets, etc.) has 1500 range which is longer than Xerath , and gives a sizable slow and armor shred to first thing hit, hitting minions knocks them back and slows whatever they hit as well., learning to use this properly is key to mastering Sion.

Global sound ult, at max cdr (40%) its 36 seconds and is great for all you movement needs. Does damage to towers, ignores unit collision with non-enemy champions, and in general is your initiation button. You can cancel it early to do an aoe ground pound stop and it will still deal the same damage, just in that area. If you have over 950 movespeed it actually slows you.

Decide beforehand on how you are going to use your ult, either it will be to initiate (you can charge your W during ult) by ramming into a squishy and following with E and Q, short ult into their team (has a mini effect), peel by ulting their initiation, or finish off a tower they are sitting under (Sion's ult does damage to towers). From there just be a general wall and get up into the carries faces/peel for your teammates.