How the Build works

Currently your (E) scales damage up by 1 per soul. And it no longer has a cooldown timer, this means that you can instead scale up your magic damage by 3 with runans, attack speed becomes your friend.

Having Spelldamage lifesteal means you become more tanky as long as you can attack.

15% * 3(=45% lifesteal) if you are hitting 3 targets suddenly becomes very worthwhile.
Add an extra 25% onto that afterwards Another (11.5%) you are healing 50% of your attack Damage add Talent tree and you can see where this is going.

Thornmail (Required against ad champs) basically kills them and heals you back a portion of the damage they do to you, as well since you have eternal scaling armour you just become more tanky by them killing themselves against you.

Wiits end is your magic resist (You get fair magic resist) use this item as early as you can because it boosts your attack rate and thus your maximum damage output as well as making you tanky.

Guinsoo (3 Attacks not enough) make sure it calculates out rights your E is on hit so this doubles every 2 attacks you E (Think an extra forth attack each attack for free) since your 3 shots I do believe count as 3 not 1.

Spiritt visage because you don't run fast and are vulnerable to magic dealers.