Welcome to my Sion build.
This build is focused for solo lane you'll love this build specialy in 3v3.
They'll hardly kill even if in 3v1

don't forget that you are mainly a tank so you have to boost your defense with runes I prefer to boost magic resist cause it is your lowest defensive stat and I think the most important in my honest opinion, you can swap some magic resist with crit that will help you to last hit minions that is your main aim in early/mid game

I tryed a lots of way to set masteries on sion but this is the most usefull because you'll need to stay in lane and let your enrage to stack the unlimited health pool(and unlimited attack damage from the atma's impaler)

the first item you'll have to buy is the vampiric scepter that allow you to stay in lane for more often as possible they as normal buy shoes you can choose between berserker's greave and ninja tabi if in the other team there are lots of attack damage heroes


CORE ITEM

Atma's impaler
Phathom dancer
Executioner's calling
Berserker's grave/Ninja tabi/Mercury'd treads

then you can choose between going for Thornmail and Force of nature to have a balanced armor and magic resist one of them and a Warmog's armor witch will allow you to stack other healt and you'll be realy unstoppable

PROS:

great killer
longer game stronger sion
great healer when ulti is on
low ulti cd
unlimited health can solo baron with ulti at level 3

CONS:

not good for fb
need to last hit minions in early game or it will be hard to play good
vulnerable to exaust and miss chance

if you are playing in the 3v3 map don't go bot for fb with some luck all your enemy will go bot so you'll be able to kill the first wave without your other enemy champ in your lane so you will be at level 2 and able to stun him when he will be at level 1.


but if you are playing 5v5 play defensive if alone and focus on last hitting witch is a must for this champion (ps avoid playing with other champ who need to last him minions for example veigar, nasus, ecc