This build of Sion will show you how to make max dmg with Ap Sion. You will smash your opponents 1 by 1 but you will have any problems in teamfights.. all is write down . If you are interested into .. READ IT !!!

Marks - magic penetration runes are important because you will deal more damage with spells.
Seals - i choose armor because at start i want to have heavy protect from attack damage champions.
Glyphs - i chose magic resist because i want to don't have problems with mages at start.

This masteries works pretty well with my summoner spells. If you want you can choose ghost before flash if you think its better and you can put 1 point to ghost and not to flash.

Doran's Ring : Its awesome for first item because you get enough mana regen ! When you get second doran's ring you can spam all your spells without caring about your mana.
Mejai's Soulstealer : This item is good only if you are owning your line.
Lich Bane : Very important item . The sheen effect help Sion too much.
Death's Cap : Gives awesome ability power and effect.
Ionian Boots of lucidity : Cd effect by 15% is important for Sion.
Abyssal Scepter : Magic resist and 20 magic pen.
Zhonya's Hourglass : Armor , Ability and pretty good effect.

In the begging i start with "Cryptic gaze" because it stuns enemy target.The most important spell is "Death's Caress"-it deals high damage and give you shield."Enrage"gives you bonus hp[1st level on spell +1 hp; 2nd level on spell +1.5 hp; 3rd level on spell +2hp; 4rt level on spell +2.5hp; 5th level on spell +3hp per mob."Cannibalism" gives you attack speed and lifesteal , on every hit you heal surrounding allies.

Flash:
1.You can run and save yourself.
2.You can go closer to anyone and kill him if he ran on low hp.
Ignite:
1.You decrease heal by 50% which is good vs Warwick and another heroes with big lifesteal .
2.You can finish target.

Early game : At start you need to farm mobs by last hitting.
Mid game: If you are feeded you can smash your opponents 1 by 1 and continue farming.
Late game : You are enough feed to jump first in teamfights and to make the game.

You farm mobs with your second spell at mid and late game.
At start you need to be a good last hitter.

Sion should be able to take one person out by himself. Sion is not a great character in team battles as he does burst damage, but he deals huge amounts of damage when he does attack.In teamfights you can hit all with your shield and stun the carry from another team.