Starting Items

Recommendation

I usually start with Boots and 3 Health Potions. This makes me pretty mobile. I'm able to dodge skillshots very well and also for the 3 Health Potions will help me to stay as long as possible in lane.

Alternative:

or

Alternatively you can start either with Doran's Ring, if you prefere to have more Health, Manaregeneration and some more Ability Power for the start, or, if you want to rush a Rod of Ages, with a Sapphire Crystal.

Early Game

Catalyst the Protector( )

When Early Game ends you should have finished your Catalyst the Protector and aswell upgraded your Boots to Sorcerer's Shoes. Don't forget to spend some gold to Sight Wards. This rather help you to prevent incoming ganks by the enemy's Jungler. If your early game works very well and you're able to farm a lot and get some kills, you can get an early Needlessly Large Rod.

Mid Game

Either if you could achieve your Needlessly Large Rod or not, finish your Rod of Ages as fast as possible. This should happen before 20min. Next item you will get is a Rabadon's Deathcap which scales relatively well with the Ability Power you will receive from Rod of Ages

Mid Game

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Now you have to decide with path you will go:

Case 1: The enemy team stacks Magic Resistance and you feel able to survive the full duration of your ultimate Soul Shackles in the enemy's rows.
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Case 2: The enemy team has heavy damage and is able to nuke you down before the full duration of your ultimate ticks you have to stack some more defense first.
> >

Final Build

Your final build should look like this. Sorcerer's Shoes will give you together with Void Staff a quite good ammount of Magic Penetration. Rod of Ages and Banshee's Veil provides you a good survivability thanks to the Health it provides and also to the spellblock you get every 30s. Armor and Magic Resistance you will get from Banshee's Veil and Zhonya's Hourglass.