Firstly I have to say,

I do not build champions Focusing on "The meta game" so if this goes against anything you consider to be proper for the current meta game; I'm sorry, but now on to this build.

This build is unique compared to the other ones, because it will have you playing a more cautious support type role until the later game when you will become a very effective at burst ambushes.

Now if this is a play style you're interested in read on and give it a shot.

This is a pretty straight forward Rune page for my AP champions

Marks - Magic Penetration Marks

These runes help you to keep your damage up until you can afford the Void staff to increase your damage.

Seals - Flat armor or Magic Resist

Now I have a problem with my Seals I have two pages set up one with Flat Armor Runes and the other with Magic Resist, I uses Armor most of the the time (Helps you survive if you do get jumped early, as most of the time it will be by AD champs) but if you are playing against a lot of AP champions or you just have trouble playing against AP champs you can use Magic Resist here, but I do recommend Armor over MR

Gylphs - Ability Power per Level

AP champion = AP Glyphs and AP per level glyphs are just more effective scaling into the late game.

Quints - Ability Power per Level

Same as Glyphs

To start off here is a list of the finishing items and why you have them:

Boots of Swiftness

Ahri is slow plain and simple, and if you want to chase down champs you've got to be quick. and the penetration from sorcerers shoes isn't worth the drop in speed.

Void Staff

Adding some much needed AP to increase your damage output, and some magic penetration.

Rabadon's Deathcap

Standard Ap Champion item, and getting it earlier better than later helps you to keep the pressure on as players start to build HP and MR. and the bonus AP it adds is great.

Rylais Crystal Scepter

This is one of my favorite items for all AP champs, its great all round, adds some AP adds some Health and adding a slow effect to abilities is awesome to help you chase down champs or escape.

Will of the Ancients

Now I know most builds rush getting some Spell vamp but really if you play cautiously it isn't necessary until later when you have to stay in the midst of a fight for a longer period of time. (Team fights near an enemy tower)

Archangel's Staff

Now this item is another great piece because it fixes a major weakness of Ahri's (Bad Mana Regen/Mana capacity) but the time when i get it depends on whether I am dominating early and want extra mana/ap to keep it up, or if i'm not doing as great and should save up to get more of the all round items.

The bread and butter of this build is HIT & RUN and that means you will be using Orb Of Deception (Q) to harass, and when you can line it up Charm (E) to pull them into range, and by then Orb of Deception is ready again and you hit them with it and Fox Fire (W).

The main source of damage for this build will be Orb of Deception, so level it as quickly as possible, Charm is used just to pull enemies away from towers, allies, anything that helps them, Fox Fire is used to increase your damage output as you dash into a fight, pull a champion away, or just to discourage a champion from chasing you, and of course your ultimate can be used either offensively (Chasing down a champion to ensure a kill, or maneuvering closer to a champion to ensure charm hits bringing them back to a fight) or defensively (Using it's blink to put a piece of the environment between you and the opponents, or to put out some damage and put some distance between them and you.)

Now the reason Fox fire is more of a focus then Charm is because the Mana scale on leveling charm makes it too expensive to cast at the higher levels, and this build is a little mana starved early game, and fox fire remains 60 mana at all levels but increases its damage output with every upgrade. So take it and level up Spirit rush when you can.

Clarity - having clarity helps to keep you in the lane as long as possible, and while it doesn't scale into the late game, it is still nice to be able to add some mana to your pool if you need it.

Ghost - Extra speed when you need it, whether that is to run away from a gank, or to chase down a champion with low hp, this is just one of those spells that I really like to use.

Now ghost CAN be swapped out for flash if you prefer it over ghost, but remember that it is a long cool down, and her ultimate gives you 3 flashes on a minute cool down.

CONS

Not a HIGH damage build (Once abilities are popped damage output drops)
Cautious play style is required (Squishy champ until late build)
Not a sustained fighter (Hit and Run)

PROS

Great for kiting (Fox Fire is built for this)
Can pick off most solo champs (Charm is great in a 1v1)
A Pain to Catch/Kill (Charm and Spirit Rush are great for escaping)
Controls Team fights

WIP