This build generally works well if you are able to secure a kill before you die. Coming from an anivia player, I was already used to burst damage and already familiar with the item Mejai's Soulstealer . Most people think that this item is only good if you are getting kills or if your team is winning. That is completely untrue, for the most part. The fact that this item gives you stacks on assist means that even if your team is losing a team fight, and you happen to get a kill or assist and escape alive, you'll still be getting stronger, for a low price of an initial 1235 gold. This means that this one item can change the entire momentum of the game, as long as you simply survive and play carefully. This build is definitely not for new Ahri players unless you're naturally amazing with her early game.

The basic idea of this build is to rush soulstealers, giving you the early game advantage if you receive first blood. Following the AP rush, you then recieve cooldown reduction, spell vamp, and overall survivability.

This because ahri often dashes in and out of team fights, so maximizing killpower and survivability is essential. This is achieved by AP boos, cooldown reduction, armor, magic resist, spell vamp, and some health and mana boost.

Quints of Potenct are pretty essential in this build for the early game advantage needed for soulstealers.

Core items.
Should always be rushed first unless you're being melted. If this is true, rush shoes that will increase your survivability, either merc treads or ninja tabi.
Cooldown reduction for that ultimate burst power.
Spell vamp and ability power
Cooldown Reduction, Mana, and armor
AP boost, the UNIQUE passive works wonders with Mejai's Soulstealer, as it multiplies the amount of ap you receive from each of the stacks
Save last for extra ability power, magic resist, health, and speed boost